1 files changed, 4 insertions, 2 deletions
diff --git a/mdk-stage1/dietlibc/lib/toupper.c b/mdk-stage1/dietlibc/lib/toupper.c
index c048e60bb..f7d415e95 100644
--- a/mdk-stage1/dietlibc/lib/toupper.c
+++ b/mdk-stage1/dietlibc/lib/toupper.c
@@ -1,6 +1,8 @@
 #include <ctype.h>
 
-inline int toupper(int c) {
-  return (c>='a' && c<='z')?c-'a'+'A':c;
+inline int toupper(int ch) {
+  if ( (unsigned int)(ch - 'a') < 26u )
+    ch += 'A' - 'a';
+  return ch;
 }