diff --git a/parso/python/normalizer.py b/parso/python/normalizer.py
index 09f6416..8e330a4 100644
--- a/parso/python/normalizer.py
+++ b/parso/python/normalizer.py
@@ -97,6 +97,7 @@ class Context(object):
         self.node = node
         self.blocks = []
         self.parent_context = parent_context
+        self.used_names = dict()
 
     def is_async_funcdef(self):
         # Stupidly enough async funcdefs can have two different forms,
@@ -387,6 +388,10 @@ class ErrorFinder(Normalizer):
                 self._add_indentation_error(message, spacing)
             else:
                 self._add_syntax_error('invalid syntax', leaf)
+        elif leaf.type == 'name':
+            if leaf.value == '__debug__' and leaf.is_definition():
+                message = 'assignment to keyword'
+                self._add_syntax_error(message, leaf)
         elif leaf.type == 'string':
             if 'b' in leaf.string_prefix.lower() \
                     and any(c for c in leaf.value if ord(c) > 127):
diff --git a/test/test_python_errors.py b/test/test_python_errors.py
index 943d55b..a1e27bd 100644
--- a/test/test_python_errors.py
+++ b/test/test_python_errors.py
@@ -81,7 +81,8 @@ def test_indentation_errors(code, positions):
         'from .__future__ import whatever',
         'def f(x=3, y): pass',
         'lambda x=3, y: x',
-        #'__debug__ = 1'
+        '__debug__ = 1',
+        'with x() as __debug__: pass',
         # Mostly 3.6 relevant
         '[]: int',
         '[a, b]: int',