diff --git a/include/clang/Basic/ConvertUTF.h b/include/clang/Basic/ConvertUTF.h
index 2d13d2c..7e04a12 100644
--- a/include/clang/Basic/ConvertUTF.h
+++ b/include/clang/Basic/ConvertUTF.h
@@ -217,7 +217,7 @@
   if (*source == sourceEnd)
     return sourceExhausted;
   unsigned size = getNumBytesForUTF8(**source);
-  if (size > sourceEnd - *source)
+  if ((ptrdiff_t)size > sourceEnd - *source)
     return sourceExhausted;
   return ConvertUTF8toUTF32(source, *source + size, &target, target + 1, flags);
 }