diff --git a/mrbgems/mruby-sprintf/src/sprintf.c b/mrbgems/mruby-sprintf/src/sprintf.c
index d4290ec59..b5f9284d9 100644
--- a/mrbgems/mruby-sprintf/src/sprintf.c
+++ b/mrbgems/mruby-sprintf/src/sprintf.c
@@ -720,10 +720,10 @@ retry:
           CHECK(n);
           memcpy(buf+blen, c, n);
           blen += n;
-          FILL(' ', width-1);
+          if (width>0) FILL(' ', width-1);
         }
         else {
-          FILL(' ', width-1);
+          if (width>0) FILL(' ', width-1);
           CHECK(n);
           memcpy(buf+blen, c, n);
           blen += n;