diff --git a/lark/parsers/resolve_ambig.py b/lark/parsers/resolve_ambig.py
index f60a3f0..9c859b5 100644
--- a/lark/parsers/resolve_ambig.py
+++ b/lark/parsers/resolve_ambig.py
@@ -30,11 +30,15 @@ def _compare_priority(tree1, tree2):
     tree1.iter_subtrees()
 
 def _compare_drv(tree1, tree2):
-    try:
-        rule1, rule2 = tree1.rule, tree2.rule
-    except AttributeError:
-        # Probably non-trees, or user trees that weren't created by the parse (better way to distinguish?)
+    rule1 = getattr(tree1, 'rule', None)
+    rule2 = getattr(tree2, 'rule', None)
+
+    if None == rule1 == rule2:
         return compare(tree1, tree2)
+    elif rule1 is None:
+        return -1
+    elif rule2 is None:
+        return 1
 
     assert tree1.data != '_ambig'
     assert tree2.data != '_ambig'