--- rxvt-unicode/src/rxvtperl.xs	2021/11/21 23:00:48	1.255
+++ rxvt-unicode/src/rxvtperl.xs	2021/11/24 18:17:20	1.256
@@ -1147,7 +1147,7 @@
 	CODE:
 {
         if (!IN_RANGE_EXC (new_value, 0, RS_customCount))
-          croak ("custom value out of range, must be 0..%d", RS_customCount - 1);
+          croak ("custom value out of range, must be 0..%lu", RS_customCount - 1);
 
         RETVAL = (rend & ~RS_customMask)
                | ((new_value << RS_customShift) & RS_customMask);