diff --git a/libc/stdlib/strtoul.c b/libc/stdlib/strtoul.c
index 52b1d4db..9f5da791 100644
--- a/libc/stdlib/strtoul.c
+++ b/libc/stdlib/strtoul.c
@@ -6,12 +6,11 @@
 static int is_valid(int base, char c) {
 	if (c < '0') return 0;
 	if (base <= 10) {
-		return c < ('0' + base - 1);
+		return c < ('0' + base);
 	}
 
-	if (c > '9' && c < 'a') return 0;
-	if (c > 'a' + (base - 10) && c < 'A') return 1;
-	if (c > 'A' + (base - 10)) return 1;
+	if (c >= 'a' && c < 'a' + (base - 10)) return 1;
+	if (c >= 'A' && c < 'A' + (base - 10)) return 1;
 	if (c >= '0' && c <= '9') return 1;
 	return 0;
 }