diff --git a/contrib/opencensus-ext-flask/opencensus/ext/flask/flask_middleware.py b/contrib/opencensus-ext-flask/opencensus/ext/flask/flask_middleware.py
index ebd8d24ff..fcbdc41e7 100644
--- a/contrib/opencensus-ext-flask/opencensus/ext/flask/flask_middleware.py
+++ b/contrib/opencensus-ext-flask/opencensus/ext/flask/flask_middleware.py
@@ -176,9 +176,11 @@ def _after_request(self, response):
 
         try:
             tracer = execution_context.get_opencensus_tracer()
-            tracer.add_attribute_to_current_span(
-                HTTP_ROUTE, flask.request.url_rule.rule
-            )
+            url_rule = flask.request.url_rule
+            if url_rule is not None:
+                tracer.add_attribute_to_current_span(
+                    HTTP_ROUTE, url_rule.rule
+                )
             tracer.add_attribute_to_current_span(
                 HTTP_STATUS_CODE,
                 response.status_code
diff --git a/contrib/opencensus-ext-flask/tests/test_flask_middleware.py b/contrib/opencensus-ext-flask/tests/test_flask_middleware.py
index 6cb59f177..bcd11a506 100644
--- a/contrib/opencensus-ext-flask/tests/test_flask_middleware.py
+++ b/contrib/opencensus-ext-flask/tests/test_flask_middleware.py
@@ -19,6 +19,7 @@
 
 from google.rpc import code_pb2
 import flask
+from werkzeug.exceptions import NotFound
 import mock
 
 from opencensus.ext.flask import flask_middleware
@@ -311,6 +312,48 @@ def test__after_request_sampled(self):
             self.assertEqual(span.attributes, expected_attributes)
             assert isinstance(span.parent_span, base.NullContextManager)
 
+    def test__after_request_invalid_url(self):
+        flask_trace_header = 'traceparent'
+        trace_id = '2dd43a1d6b2549c6bc2a1a54c2fc0b05'
+        span_id = '6e0c63257de34c92'
+        flask_trace_id = '00-{}-{}-00'.format(trace_id, span_id)
+
+        app = self.create_app()
+        flask_middleware.FlaskMiddleware(
+            app=app,
+            sampler=samplers.AlwaysOnSampler()
+        )
+
+        context = app.test_request_context(
+            path='/this-url-does-not-exist',
+            headers={flask_trace_header: flask_trace_id}
+        )
+
+        with context:
+            app.preprocess_request()
+            tracer = execution_context.get_opencensus_tracer()
+            self.assertIsNotNone(tracer)
+
+            span = tracer.current_span()
+
+            try:
+                rv = app.dispatch_request()
+            except NotFound as e:
+                rv = app.handle_user_exception(e)
+            app.finalize_request(rv)
+
+            # http.route should not be set
+            expected_attributes = {
+                'http.host': u'localhost',
+                'http.method': u'GET',
+                'http.path': u'/this-url-does-not-exist',
+                'http.url': u'http://localhost/this-url-does-not-exist',
+                'http.status_code': 404
+            }
+
+            self.assertEqual(span.attributes, expected_attributes)
+            assert isinstance(span.parent_span, base.NullContextManager)
+
     def test__after_request_blacklist(self):
         flask_trace_header = 'traceparent'
         trace_id = '2dd43a1d6b2549c6bc2a1a54c2fc0b05'