diff --git a/client/util.c b/client/util.c
index af8141da..7e6b4074 100644
--- a/client/util.c
+++ b/client/util.c
@@ -620,7 +620,7 @@ int hextobinarray(char *target, char *source)
         else if (x >= 'A' && x <= 'F')
             x -= 'A' - 10;
         else {
-        	printf("Discovered unknown character %c %d at idx %d of %s\n", x, x, source - start, start);
+        	printf("Discovered unknown character %c %d at idx %tu of %s\n", x, x, source - start, start);
             return 0;
         }
         // output