diff --git a/llvm/lib/VMCore/AsmWriter.cpp b/llvm/lib/VMCore/AsmWriter.cpp
index 669d308b1a1c..aedb86b3eed0 100644
--- a/llvm/lib/VMCore/AsmWriter.cpp
+++ b/llvm/lib/VMCore/AsmWriter.cpp
@@ -100,7 +100,11 @@ static void PrintLLVMName(raw_ostream &OS, StringRef Name, PrefixType Prefix) {
   bool NeedsQuotes = isdigit(Name[0]);
   if (!NeedsQuotes) {
     for (unsigned i = 0, e = Name.size(); i != e; ++i) {
-      char C = Name[i];
+      // By making this unsigned, the value passed in to isalnum will always be
+      // in the range 0-255.  This is important when building with MSVC because
+      // its implementation will assert.  This situation can arise when dealing
+      // with UTF-8 multibyte characters.
+      unsigned char C = Name[i];
       if (!isalnum(C) && C != '-' && C != '.' && C != '_') {
         NeedsQuotes = true;
         break;