diff --git a/jedi/evaluate/__init__.py b/jedi/evaluate/__init__.py
index 7579b5db..26eacbe9 100644
--- a/jedi/evaluate/__init__.py
+++ b/jedi/evaluate/__init__.py
@@ -198,8 +198,13 @@ class Evaluator(object):
             # The implicit tuple in statements.
             return [iterable.Array(self, element, pr.Array.TUPLE)]
         else:
-            left, operator, right = element.children
-            return precedence.calculate(self, self.eval_element(left), operator,
+            if element.children[0] == 'not':
+                left = []
+                operator, right = element.children
+            else:
+                left, operator, right = element.children
+                left = self.eval_element(left)
+            return precedence.calculate(self, left, operator,
                                         self.eval_element(right))
 
     def _eval_atom(self, atom):
diff --git a/jedi/evaluate/stdlib.py b/jedi/evaluate/stdlib.py
index d94b7d68..4b391768 100644
--- a/jedi/evaluate/stdlib.py
+++ b/jedi/evaluate/stdlib.py
@@ -164,11 +164,10 @@ def builtins_reversed(evaluator, obj, params):
     return [er.Instance(evaluator, obj, objects)]
 
 
-def builtins_isinstance(evaluator, obj, params):
-    obj = _follow_param(evaluator, params, 0)
-    raw_classes = _follow_param(evaluator, params, 1)
+@argument_clinic('obj, type, /')
+def builtins_isinstance(evaluator, objects, types):
     bool_results = set([])
-    for o in obj:
+    for o in objects:
         try:
             mro_func = o.py__class__(evaluator).py__mro__
         except AttributeError:
@@ -179,7 +178,7 @@ def builtins_isinstance(evaluator, obj, params):
 
         mro = mro_func(evaluator)
 
-        for cls_or_tup in raw_classes:
+        for cls_or_tup in types:
             if cls_or_tup.is_class():
                 bool_results.add(cls_or_tup in mro)
             else: