Improve error message

ref [Bug #13761]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59452 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 1 insertions, 1 deletions

diff --git a/sprintf.c b/sprintf.c
index 0d685ddbaf..52771eb52a 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -521,7 +521,7 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
 
 	for (t = p; t < end && *t != '%'; t++) ;
 	if (t + 1 == end) {
-	    rb_raise(rb_eArgError, "incomplete format specifier");
+	    rb_raise(rb_eArgError, "incomplete format specifier; use %%%% (double %%) instead");
 	}
 	PUSH(p, t - p);
 	if (coderange != ENC_CODERANGE_BROKEN && scanned < blen) {