diff --git a/src/fmt_fp.c b/src/fmt_fp.c
index 783cc6c02..7028d3aac 100644
--- a/src/fmt_fp.c
+++ b/src/fmt_fp.c
@@ -379,7 +379,7 @@ mrb_float_to_str(mrb_state *mrb, mrb_value flo, const char *fmt)
 mrb_value
 mrb_float_to_str(mrb_state *mrb, mrb_value flo, const char *fmt)
 {
-  char buf[24];
+  char buf[25];
 
   snprintf(buf, sizeof(buf), fmt, mrb_float(flo));
   return mrb_str_new_cstr(mrb, buf);