diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
index 74efcfb25a8fad7c3e87b92b00886a1bac7fc177..429780eaea33efe3856d47142d4209792bce2d21 100644
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -2749,7 +2749,7 @@ uint32_t Lexer::tryReadUCN(const char *&StartPtr, const char *SlashLoc,
 
   if (Result) {
     Result->setFlag(Token::HasUCN);
-    if (CurPtr - StartPtr == NumHexDigits + 2)
+    if (CurPtr - StartPtr == (ptrdiff_t)NumHexDigits + 2)
       StartPtr = CurPtr;
     else
       while (StartPtr != CurPtr)