diff --git a/mistral/tests/unit/test_expressions.py b/mistral/tests/unit/test_expressions.py
index 5149222b8..2d0a51733 100644
--- a/mistral/tests/unit/test_expressions.py
+++ b/mistral/tests/unit/test_expressions.py
@@ -77,6 +77,15 @@ class YaqlEvaluatorTest(base.BaseTest):
         self.assertEqual('3', self._evaluator.evaluate('str($)', '3'))
         self.assertEqual('3', self._evaluator.evaluate('str($)', 3))
 
+    def test_function_len(self):
+        self.assertEqual(3, self._evaluator.evaluate('$.len()', 'hey'))
+        data = [{'some': 'thing'}]
+
+        self.assertEqual(
+            1,
+            self._evaluator.evaluate('$[$.some = thing].len()', data)
+        )
+
     def test_validate(self):
         self._evaluator.validate('abc')
         self._evaluator.validate('1')
diff --git a/mistral/yaql_utils.py b/mistral/yaql_utils.py
index 46593edfe..e89ab3c48 100644
--- a/mistral/yaql_utils.py
+++ b/mistral/yaql_utils.py
@@ -13,8 +13,6 @@
 #    limitations under the License.
 
 import collections
-import six
-import types
 
 import yaql
 from yaql import context
@@ -29,21 +27,21 @@ def create_yaql_context():
 
 
 def _register_functions(yaql_ctx):
-    yaql_ctx.register_function(_string_and_iterable_length, 'len')
-    yaql_ctx.register_function(_generator_length, 'len')
+    yaql_ctx.register_function(_sized_length, 'len')
+    yaql_ctx.register_function(_iterable_length, 'len')
     yaql_ctx.register_function(to_str, 'str')
 
 
 # Additional convenience YAQL functions.
 
 
-@context.EvalArg('a', arg_type=(six.string_types, collections.Iterable))
-def _string_and_iterable_length(a):
+@context.EvalArg('a', arg_type=collections.Sized)
+def _sized_length(a):
     return len(a)
 
 
-@context.EvalArg('a', arg_type=types.GeneratorType)
-def _generator_length(a):
+@context.EvalArg('a', arg_type=collections.Iterable)
+def _iterable_length(a):
     return sum(1 for i in a)