diff --git a/lib/Support/StringRef.cpp b/lib/Support/StringRef.cpp
index c33ca6a2ebf..46f26b242aa 100644
--- a/lib/Support/StringRef.cpp
+++ b/lib/Support/StringRef.cpp
@@ -59,7 +59,7 @@ int StringRef::compare_numeric(StringRef RHS) const {
           break;
       }
     }
-    return Data[I] < RHS.Data[I] ? -1 : 1;
+    return (unsigned char)Data[I] < (unsigned char)RHS.Data[I] ? -1 : 1;
   }
   if (Length == RHS.Length)
     return 0;
diff --git a/unittests/ADT/StringRefTest.cpp b/unittests/ADT/StringRefTest.cpp
index de5c349cd60..7e4d0dcd413 100644
--- a/unittests/ADT/StringRefTest.cpp
+++ b/unittests/ADT/StringRefTest.cpp
@@ -72,6 +72,7 @@ TEST(StringRefTest, StringOps) {
   EXPECT_EQ( 0, StringRef("10a").compare_numeric("10a"));
   EXPECT_EQ( 1, StringRef("2").compare_numeric("1"));
   EXPECT_EQ( 0, StringRef("llvm_v1i64_ty").compare_numeric("llvm_v1i64_ty"));
+  EXPECT_EQ( 1, StringRef("\xFF").compare_numeric("\1"));
 }
 
 TEST(StringRefTest, Operators) {