diff --git a/sprintf.c b/sprintf.c
index 0438b5e32c..92738783eb 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -55,8 +55,9 @@ sign_bits(int base, const char *p)
 
 #define CHECK(l) do {\
     int cr = ENC_CODERANGE(result);\
-    while (blen + (l) >= bsiz) {\
+    while ((l) >= bsiz - blen) {\
 	bsiz*=2;\
+	if (bsiz<0) rb_raise(rb_eArgError, "too big specifier");\
     }\
     rb_str_resize(result, bsiz);\
     ENC_CODERANGE_SET(result, cr);\