diff --git a/src/Pattern.php b/src/Pattern.php
index 9b625fa..9889602 100644
--- a/src/Pattern.php
+++ b/src/Pattern.php
@@ -292,7 +292,15 @@ protected function compileParserArgs(Ast $args) : array {
         return $compiled;
     }
 
-    private function compileAnonymousFunctionArg(array $arg) : \Closure {
+    private function compileAnonymousFunctionArg($arg) : \Closure {
+        if ($arg instanceof Ast) {
+            $arg = $arg->unwrap();
+        }
+
+        if (!is_array($arg)) {
+            throw new InvalidArgumentException('$arg should be an array or instance of Yay\Ast');
+        }
+
         $arglist = implode('', $arg['args']);
         $body = implode('', $arg['body']);
         $source = "<?php\nreturn static function({$arglist}){\n{$body}\n};";
diff --git a/tests/phpt/midrule.phpt b/tests/phpt/midrule.phpt
new file mode 100644
index 0000000..55d6375
--- /dev/null
+++ b/tests/phpt/midrule.phpt
@@ -0,0 +1,42 @@
+--TEST--
+Check that midrule works
+--FILE--
+<?php
+
+macro {
+    ·chain(
+        matched,
+        ·midrule(function($stream) {
+            $index = $stream->index();
+
+            if (in_array(strtolower($stream->current()), ["true", "false", "null"])) {
+                return new \Yay\Error(null, null, $stream->last());
+            }
+
+            return new \Yay\Ast;
+        }),
+        ·ns()·ns
+    )
+} >> {
+    replaced ·ns
+}
+
+matched true
+matched false
+matched null
+
+matched strtoupper
+matched ucwords
+
+?>
+--EXPECTF--
+<?php
+
+matched true
+matched false
+matched null
+
+replaced strtoupper
+replaced ucwords
+
+?>