diff --git a/llvm/lib/Object/COFFObjectFile.cpp b/llvm/lib/Object/COFFObjectFile.cpp
index 0b7ee34..ca90e0e 100644
--- a/llvm/lib/Object/COFFObjectFile.cpp
+++ b/llvm/lib/Object/COFFObjectFile.cpp
@@ -267,7 +267,7 @@
   }
 
   if (symb->StorageClass == COFF::IMAGE_SYM_CLASS_EXTERNAL)
-    ret = ::toupper(ret);
+    ret = ::toupper(static_cast<unsigned char>(ret));
 
   Result = ret;
   return object_error::success;