diff --git a/include/llvm/ADT/StringExtras.h b/include/llvm/ADT/StringExtras.h
index a0b3fe7a06c..a152f4d3c2a 100644
--- a/include/llvm/ADT/StringExtras.h
+++ b/include/llvm/ADT/StringExtras.h
@@ -141,7 +141,7 @@ void SplitString(StringRef Source,
 // better: http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx
 //   X*33+c -> X*33^c
 static inline unsigned HashString(StringRef Str, unsigned Result = 0) {
-  for (unsigned i = 0, e = Str.size(); i != e; ++i)
+  for (StringRef::size_type i = 0, e = Str.size(); i != e; ++i)
     Result = Result * 33 + (unsigned char)Str[i];
   return Result;
 }