diff --git a/lark/reconstruct.py b/lark/reconstruct.py
index 46dc6c5..2e51e93 100644
--- a/lark/reconstruct.py
+++ b/lark/reconstruct.py
@@ -34,7 +34,8 @@ class WriteTokensTransformer(Transformer_InPlace):
         for sym in meta.orig_expansion:
             if is_discarded_terminal(sym):
                 t = self.tokens[sym.name]
-                assert isinstance(t.pattern, PatternStr)
+                if not isinstance(t.pattern, PatternStr):
+                    raise NotImplementedError("Reconstructing regexps not supported yet: %s" % t)
                 to_write.append(t.pattern.value)
             else:
                 x = next(iter_args)
diff --git a/tests/test_reconstructor.py b/tests/test_reconstructor.py
index ec264a0..5111d2b 100644
--- a/tests/test_reconstructor.py
+++ b/tests/test_reconstructor.py
@@ -40,9 +40,9 @@ class TestReconstructor(TestCase):
     def test_starred_group(self):
 
         g = """
-        start: (rule | _NL)*
+        start: (rule | NL)*
         rule: WORD ":" NUMBER
-        _NL: /(\\r?\\n)+\s*/
+        NL: /(\\r?\\n)+\s*/
         """ + common
 
         code = """