diff --git a/src/librustc/middle/stability.rs b/src/librustc/middle/stability.rs
index 3717ee7143c..c726885337e 100644
--- a/src/librustc/middle/stability.rs
+++ b/src/librustc/middle/stability.rs
@@ -765,7 +765,9 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
                 }
             }
             EvalResult::Unmarked => {
-                span_bug!(span, "encountered unmarked API: {:?}", def_id);
+                // The API could be uncallable for other reasons, for example when a private module
+                // was referenced.
+                self.sess.delay_span_bug(span, &format!("encountered unmarked API: {:?}", def_id));
             }
         }
     }
diff --git a/src/test/ui/stability-in-private-module.rs b/src/test/ui/stability-in-private-module.rs
new file mode 100644
index 00000000000..f12e9198b0d
--- /dev/null
+++ b/src/test/ui/stability-in-private-module.rs
@@ -0,0 +1,4 @@
+fn main() {
+    let _ = std::thread::thread_info::current_thread();
+    //~^ERROR module `thread_info` is private
+}
diff --git a/src/test/ui/stability-in-private-module.stderr b/src/test/ui/stability-in-private-module.stderr
new file mode 100644
index 00000000000..c3edd62a15e
--- /dev/null
+++ b/src/test/ui/stability-in-private-module.stderr
@@ -0,0 +1,9 @@
+error[E0603]: module `thread_info` is private
+  --> $DIR/stability-in-private-module.rs:2:26
+   |
+LL |     let _ = std::thread::thread_info::current_thread();
+   |                          ^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0603`.