1 files changed, 1 insertions, 1 deletions
diff --git a/singles.c b/singles.c
index d8016ff..c20d0cf 100644
--- a/singles.c
+++ b/singles.c
@@ -324,7 +324,7 @@ static char n2c(int num) {
 }
 
 static int c2n(char c) {
-    if (isdigit(c))
+    if (isdigit((unsigned char)c))
         return (int)(c - '0');
     else if (c >= 'a' && c <= 'z')
         return (int)(c - 'a' + 10);