diff --git a/engine/skin.php b/engine/skin.php
index 17dcf8e..2ba35f6 100644
--- a/engine/skin.php
+++ b/engine/skin.php
@@ -159,7 +159,14 @@ class SkinContext {
         $fn = $this->ns.'\\'.$name;
         $refl = new ReflectionFunction($fn);
         $fparams = $refl->getParameters();
-        assert(count($fparams) == count($arguments) + 1, "$fn: invalid number of arguments (".count($fparams)." != ".(count($arguments) + 1).")");
+        $fparams_required_count = 0;
+        foreach ($fparams as $param) {
+            if (!$param->isDefaultValueAvailable())
+                $fparams_required_count++;
+        }
+        $given_count = count($arguments)+1;
+        assert($given_count >= $fparams_required_count && $given_count <= count($fparams),
+            "$fn: invalid number of arguments (function has ".$fparams_required_count." required arguments".(count($fparams) != $fparams_required_count ? ' and '.count($fparams).' total argumments' : '').", received ".(count($arguments) + 1).")");
 
         foreach ($fparams as $n => $param) {
             if ($n == 0)