Improve error message when subclassing non-Class

Fixes [Bug #14726]

1 files changed, 1 insertions, 1 deletions

diff --git a/class.c b/class.c
index 4b8130d4ec..7cc087f182 100644
--- a/class.c
+++ b/class.c
@@ -227,7 +227,7 @@ void
 rb_check_inheritable(VALUE super)
 {
     if (!RB_TYPE_P(super, T_CLASS)) {
-	rb_raise(rb_eTypeError, "superclass must be a Class (%"PRIsVALUE" given)",
+        rb_raise(rb_eTypeError, "superclass must be an instance of Class (given an instance of %"PRIsVALUE")",
 		 rb_obj_class(super));
     }
     if (RBASIC(super)->flags & FL_SINGLETON) {