diff --git a/clang/lib/Format/TokenAnnotator.cpp b/clang/lib/Format/TokenAnnotator.cpp
index 2785ca45e70f..16115611a5da 100644
--- a/clang/lib/Format/TokenAnnotator.cpp
+++ b/clang/lib/Format/TokenAnnotator.cpp
@@ -2391,7 +2391,8 @@ bool TokenAnnotator::mustBreakBefore(const AnnotatedLine &Line,
         Right.Next->is(tok::string_literal))
       return true;
   } else if (Style.Language == FormatStyle::LK_Cpp ||
-             Style.Language == FormatStyle::LK_ObjC) {
+             Style.Language == FormatStyle::LK_ObjC ||
+             Style.Language == FormatStyle::LK_Proto) {
     if (Left.isStringLiteral() &&
         (Right.isStringLiteral() || Right.is(TT_ObjCStringLiteral)))
       return true;
diff --git a/clang/unittests/Format/FormatTestProto.cpp b/clang/unittests/Format/FormatTestProto.cpp
index 6881af4361cf..41d210ac7cd7 100644
--- a/clang/unittests/Format/FormatTestProto.cpp
+++ b/clang/unittests/Format/FormatTestProto.cpp
@@ -135,6 +135,9 @@ TEST_F(FormatTestProto, MessageFieldAttributes) {
                "    key: 'a'  //\n"
                "  }\n"
                "];");
+  verifyFormat("optional string test = 1 [default =\n"
+               "                              \"test\"\n"
+               "                              \"test\"];");
 }
 
 TEST_F(FormatTestProto, DoesntWrapFileOptions) {