diff --git a/libcxx/include/string b/libcxx/include/string
index 9f4007afc188..840932a4d7a0 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -639,16 +639,16 @@ struct _LIBCPP_TYPE_VIS_ONLY char_traits<char>
         {return __n == 0 ? 0 : memcmp(__s1, __s2, __n);}
     static inline size_t length(const char_type* __s) {return strlen(__s);}
     static inline const char_type* find(const char_type* __s, size_t __n, const char_type& __a)
-        {return (const char_type*)memchr(__s, to_int_type(__a), __n);}
+        {return __n == 0 ? NULL : (const char_type*) memchr(__s, to_int_type(__a), __n);}
     static inline char_type* move(char_type* __s1, const char_type* __s2, size_t __n)
-        {return (char_type*)memmove(__s1, __s2, __n);}
+        {return __n == 0 ? __s1 : (char_type*) memmove(__s1, __s2, __n);}
     static inline char_type* copy(char_type* __s1, const char_type* __s2, size_t __n)
         {
             _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
-            return (char_type*)memcpy(__s1, __s2, __n);
+            return __n == 0 ? __s1 : (char_type*)memcpy(__s1, __s2, __n);
         }
     static inline char_type* assign(char_type* __s, size_t __n, char_type __a)
-        {return (char_type*)memset(__s, to_int_type(__a), __n);}
+        {return __n == 0 ? __s : (char_type*)memset(__s, to_int_type(__a), __n);}
 
     static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
@@ -685,16 +685,16 @@ struct _LIBCPP_TYPE_VIS_ONLY char_traits<wchar_t>
     static inline size_t length(const char_type* __s)
         {return wcslen(__s);}
     static inline const char_type* find(const char_type* __s, size_t __n, const char_type& __a)
-        {return (const char_type*)wmemchr(__s, __a, __n);}
+        {return __n == 0 ? NULL : (const char_type*)wmemchr(__s, __a, __n);}
     static inline char_type* move(char_type* __s1, const char_type* __s2, size_t __n)
-        {return (char_type*)wmemmove(__s1, __s2, __n);}
+        {return __n == 0 ? __s1 : (char_type*)wmemmove(__s1, __s2, __n);}
     static inline char_type* copy(char_type* __s1, const char_type* __s2, size_t __n)
         {
             _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
-            return (char_type*)wmemcpy(__s1, __s2, __n);
+            return __n == 0 ? __s1 : (char_type*)wmemcpy(__s1, __s2, __n);
         }
     static inline char_type* assign(char_type* __s, size_t __n, char_type __a)
-        {return (char_type*)wmemset(__s, __a, __n);}
+        {return __n == 0 ? __s : (char_type*)wmemset(__s, __a, __n);}
 
     static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/assign3.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/assign3.pass.cpp
index 2b192994f365..a4a0e294c262 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/assign3.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/assign3.pass.cpp
@@ -24,4 +24,5 @@ int main()
     assert(s2[0] == char(5));
     assert(s2[1] == char(5));
     assert(s2[2] == char(5));
+    assert(std::char_traits<char>::assign(NULL, 0, char(5)) == NULL);
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/copy.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/copy.pass.cpp
index e57a677b9a30..179df9e19dde 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/copy.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/copy.pass.cpp
@@ -24,4 +24,6 @@ int main()
     assert(s2[0] == char(1));
     assert(s2[1] == char(2));
     assert(s2[2] == char(3));
+    assert(std::char_traits<char>::copy(NULL, s1, 0) == NULL);
+    assert(std::char_traits<char>::copy(s1, NULL, 0) == s1);
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/find.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/find.pass.cpp
index 816b358968be..a640af28d8ac 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/find.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/find.pass.cpp
@@ -24,4 +24,5 @@ int main()
     assert(std::char_traits<char>::find(s1, 3, char(3)) == s1+2);
     assert(std::char_traits<char>::find(s1, 3, char(4)) == 0);
     assert(std::char_traits<char>::find(s1, 3, char(0)) == 0);
+    assert(std::char_traits<char>::find(NULL, 0, char(0)) == 0);
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/move.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/move.pass.cpp
index 691b28dd0db6..67f0216a28b1 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/move.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/move.pass.cpp
@@ -28,4 +28,6 @@ int main()
     assert(s1[0] == char(2));
     assert(s1[1] == char(2));
     assert(s1[2] == char(3));
+    assert(std::char_traits<char>::move(NULL, s1, 0) == NULL);
+    assert(std::char_traits<char>::move(s1, NULL, 0) == s1);
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/assign3.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/assign3.pass.cpp
index 4d53439ae897..fc1ba6e8670a 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/assign3.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/assign3.pass.cpp
@@ -25,5 +25,6 @@ int main()
     assert(s2[0] == char16_t(5));
     assert(s2[1] == char16_t(5));
     assert(s2[2] == char16_t(5));
+    assert(std::char_traits<char16_t>::assign(NULL, 0, char16_t(5)) == NULL);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/copy.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/copy.pass.cpp
index 1ef44635158c..4f66d2c2cade 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/copy.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/copy.pass.cpp
@@ -25,5 +25,7 @@ int main()
     assert(s2[0] == char16_t(1));
     assert(s2[1] == char16_t(2));
     assert(s2[2] == char16_t(3));
+    assert(std::char_traits<char16_t>::copy(NULL, s1, 0) == NULL);
+    assert(std::char_traits<char16_t>::copy(s1, NULL, 0) == s1);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/find.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/find.pass.cpp
index 0952d5a016f2..22db05b55a87 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/find.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/find.pass.cpp
@@ -25,5 +25,6 @@ int main()
     assert(std::char_traits<char16_t>::find(s1, 3, char16_t(3)) == s1+2);
     assert(std::char_traits<char16_t>::find(s1, 3, char16_t(4)) == 0);
     assert(std::char_traits<char16_t>::find(s1, 3, char16_t(0)) == 0);
+    assert(std::char_traits<char16_t>::find(NULL, 0, char16_t(0)) == 0);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/move.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/move.pass.cpp
index 1839b91b67cc..b2ae8040ec24 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/move.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/move.pass.cpp
@@ -29,5 +29,7 @@ int main()
     assert(s1[0] == char16_t(2));
     assert(s1[1] == char16_t(2));
     assert(s1[2] == char16_t(3));
+    assert(std::char_traits<char16_t>::move(NULL, s1, 0) == NULL);
+    assert(std::char_traits<char16_t>::move(s1, NULL, 0) == s1);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/assign3.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/assign3.pass.cpp
index cce0420be85b..0c31ea7ae97e 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/assign3.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/assign3.pass.cpp
@@ -25,5 +25,6 @@ int main()
     assert(s2[0] == char32_t(5));
     assert(s2[1] == char32_t(5));
     assert(s2[2] == char32_t(5));
+    assert(std::char_traits<char32_t>::assign(NULL, 0, char32_t(5)) == NULL);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/copy.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/copy.pass.cpp
index 5f23514abd34..89c22250b306 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/copy.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/copy.pass.cpp
@@ -25,5 +25,7 @@ int main()
     assert(s2[0] == char32_t(1));
     assert(s2[1] == char32_t(2));
     assert(s2[2] == char32_t(3));
+    assert(std::char_traits<char32_t>::copy(NULL, s1, 0) == NULL);
+    assert(std::char_traits<char32_t>::copy(s1, NULL, 0) == s1);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/find.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/find.pass.cpp
index 148fc0ccfcc2..d0b17999d3e1 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/find.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/find.pass.cpp
@@ -25,5 +25,6 @@ int main()
     assert(std::char_traits<char32_t>::find(s1, 3, char32_t(3)) == s1+2);
     assert(std::char_traits<char32_t>::find(s1, 3, char32_t(4)) == 0);
     assert(std::char_traits<char32_t>::find(s1, 3, char32_t(0)) == 0);
+    assert(std::char_traits<char32_t>::find(NULL, 0, char32_t(0)) == 0);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/move.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/move.pass.cpp
index 6fbc6d94fe59..8641525d92bd 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/move.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/move.pass.cpp
@@ -29,5 +29,7 @@ int main()
     assert(s1[0] == char32_t(2));
     assert(s1[1] == char32_t(2));
     assert(s1[2] == char32_t(3));
+    assert(std::char_traits<char32_t>::move(NULL, s1, 0) == NULL);
+    assert(std::char_traits<char32_t>::move(s1, NULL, 0) == s1);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/assign3.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/assign3.pass.cpp
index 4c5ff479d9f3..2a00fe06e5a2 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/assign3.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/assign3.pass.cpp
@@ -24,4 +24,5 @@ int main()
     assert(s2[0] == wchar_t(5));
     assert(s2[1] == wchar_t(5));
     assert(s2[2] == wchar_t(5));
+    assert(std::char_traits<wchar_t>::assign(NULL, 0, wchar_t(5)) == NULL);
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/copy.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/copy.pass.cpp
index 71ed6fb8a307..f6d254f4c7c5 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/copy.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/copy.pass.cpp
@@ -24,4 +24,6 @@ int main()
     assert(s2[0] == wchar_t(1));
     assert(s2[1] == wchar_t(2));
     assert(s2[2] == wchar_t(3));
+    assert(std::char_traits<wchar_t>::copy(NULL, s1, 0) == NULL);
+    assert(std::char_traits<wchar_t>::copy(s1, NULL, 0) == s1);
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/find.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/find.pass.cpp
index 49757390e022..ead3f32fcc72 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/find.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/find.pass.cpp
@@ -24,4 +24,5 @@ int main()
     assert(std::char_traits<wchar_t>::find(s1, 3, wchar_t(3)) == s1+2);
     assert(std::char_traits<wchar_t>::find(s1, 3, wchar_t(4)) == 0);
     assert(std::char_traits<wchar_t>::find(s1, 3, wchar_t(0)) == 0);
+    assert(std::char_traits<wchar_t>::find(NULL, 0, wchar_t(0)) == 0);
 }
diff --git a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/move.pass.cpp b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/move.pass.cpp
index f19fa93e0c6e..0d80446e7118 100644
--- a/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/move.pass.cpp
+++ b/libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/move.pass.cpp
@@ -28,4 +28,6 @@ int main()
     assert(s1[0] == wchar_t(2));
     assert(s1[1] == wchar_t(2));
     assert(s1[2] == wchar_t(3));
+    assert(std::char_traits<wchar_t>::move(NULL, s1, 0) == NULL);
+    assert(std::char_traits<wchar_t>::move(s1, NULL, 0) == s1);
 }