diff --git a/llvm/include/llvm/Support/YAMLTraits.h b/llvm/include/llvm/Support/YAMLTraits.h
index 656020ec49d796..a3bfa7dc46782a 100644
--- a/llvm/include/llvm/Support/YAMLTraits.h
+++ b/llvm/include/llvm/Support/YAMLTraits.h
@@ -649,7 +649,8 @@ inline bool isBool(StringRef S) {
 inline QuotingType needsQuotes(StringRef S) {
   if (S.empty())
     return QuotingType::Single;
-  if (isspace(S.front()) || isspace(S.back()))
+  if (isspace(static_cast<unsigned char>(S.front())) ||
+      isspace(static_cast<unsigned char>(S.back())))
     return QuotingType::Single;
   if (isNull(S))
     return QuotingType::Single;