diff --git a/builtins/set.def b/builtins/set.def
index c4a7001..4b885cb 100644
--- a/builtins/set.def
+++ b/builtins/set.def
@@ -693,8 +693,11 @@ set_builtin (list)
 		      return (r);
 		    }
 		}
-	      else if (change_flag (flag_name, on_or_off) == FLAG_ERROR)
+	      else if (flag_name == 'i' || change_flag (flag_name, on_or_off) == FLAG_ERROR)
 		{
+		  /* We disallow changing the 'i' flag from the set builtin, because it
+		     confuses people. Also because setting it here is a no-op, and it just
+		     messes up with the value of $- */
 		  s[0] = on_or_off;
 		  s[1] = flag_name;
 		  s[2] = '\0';