diff --git a/jedi/api/classes.py b/jedi/api/classes.py
index 6f4f2627..297329af 100644
--- a/jedi/api/classes.py
+++ b/jedi/api/classes.py
@@ -363,6 +363,10 @@ class BaseDefinition(object):
             for signature in context.get_signatures():
                 return [Definition(self._evaluator, n) for n in signature.get_param_names()]
 
+        if self.type == 'function' or self.type == 'class':
+            # Fallback, if no signatures were defined (which is probably by
+            # itself a bug).
+            return []
         raise AttributeError('There are no params defined on this.')
 
     def parent(self):
diff --git a/test/test_api/test_classes.py b/test/test_api/test_classes.py
index e2148018..b0cf2432 100644
--- a/test/test_api/test_classes.py
+++ b/test/test_api/test_classes.py
@@ -174,6 +174,12 @@ def test_completion_params(Script):
     assert [p.name for p in c.params] == ['s', 'sep']
 
 
+def test_functions_should_have_params(Script):
+    for c in Script('bool.').completions():
+        if c.type == 'function':
+            assert isinstance(c.params, list)
+
+
 def test_hashlib_params(Script, environment):
     if environment.version_info < (3,):
         pytest.skip()