diff --git a/quantities/registry.py b/quantities/registry.py
@@ -29,7 +29,8 @@ def __init__(self):
        def __getitem__(self, string):
            # This approach to avoiding arbitrary evaluation of code is based on https://stackoverflow.com/a/11952618 
            # by https://stackoverflow.com/users/567292/ecatmur
-            tree = ast.parse(string, mode="eval")
            valid = all(isinstance(node, self.whitelist) for node in ast.walk(tree))
            if valid:
                try: