diff --git a/scripts/gen_post.rb b/scripts/gen_post.rb
index e0bc10e..ae0d614 100755
--- a/scripts/gen_post.rb
+++ b/scripts/gen_post.rb
@@ -38,7 +38,10 @@ def convert_to_valid_loading_mode str
     opts = opts || {};
 
     // Default print level is errors only
-    this.print_level = opts.print_level || 1;
+    this.print_level = 1;
+    if (typeof opts.print_level === "number" && opts.print_level >= 0) {
+      this.print_level = opts.print_level;
+    }
     this.mrb = _mrb_open();
     _webruby_internal_setup(this.mrb);
   };
@@ -49,6 +52,9 @@ def convert_to_valid_loading_mode str
   WEBRUBY.prototype.run = function() {
     _webruby_internal_run(this.mrb, this.print_level);
   };
+  WEBRUBY.prototype.set_print_level = function(level) {
+    if (level >= 0) this.print_level = level;
+  };
 __EOF__
 
   if mode > 0