diff --git a/mrbgems/mruby-bigint/core/bigint.c b/mrbgems/mruby-bigint/core/bigint.c
index 2c8487152..5b3be1f6e 100644
--- a/mrbgems/mruby-bigint/core/bigint.c
+++ b/mrbgems/mruby-bigint/core/bigint.c
@@ -4035,7 +4035,7 @@ mpz_get_str(mpz_ctx_t *ctx, char *s, mrb_int sz, mrb_int base, mpz_t *x)
         }
 
         // convert to character
-        for (mp_limb b=b2; b>=base; b/=(mp_limb)base) {
+        for (mp_limb b=b2; b>=(mp_limb)base; b/=(mp_limb)base) {
           char a0 = (char)(a % base);
           if (a0 < 10) a0 += '0';
           else a0 += 'a' - 10;