diff --git a/flask/app.py b/flask/app.py
index 6baee3a6..1f3d39ce 100644
--- a/flask/app.py
+++ b/flask/app.py
@@ -14,7 +14,7 @@ from threading import Lock
 from datetime import timedelta
 from itertools import chain
 from functools import update_wrapper
-from collections import Mapping
+from collections import Mapping, deque
 
 from werkzeug.datastructures import ImmutableDict
 from werkzeug.routing import Map, Rule, RequestRedirect, BuildError
@@ -1411,27 +1411,37 @@ class Flask(_PackageBoundObject):
         If neither blueprint nor App has a suitable handler registered, returns None
         """
         exc_class, code = self._get_exc_class_and_code(type(e))
-        
-        def find_superclass(handler_map):
+
+        def find_handler(handler_map):
             if not handler_map:
-                return None
-            for superclass in exc_class.__mro__:
-                if superclass is BaseException:
-                    return None
-                handler = handler_map.get(superclass)
+                return
+            queue = deque(exc_class.__mro__)
+            # Protect from geniuses who might create circular references in
+            # __mro__
+            done = set()
+
+            while True:
+                cls = queue.popleft()
+                if cls in done:
+                    continue
+                done.add(cls)
+                handler = handler_map.get(cls)
                 if handler is not None:
-                    handler_map[exc_class] = handler  # cache for next time exc_class is raised
+                    # cache for next time exc_class is raised
+                    handler_map[exc_class] = handler
                     return handler
-            return None
-        
+
+                queue.extend(cls.__mro__)
+
         # try blueprint handlers
-        handler = find_superclass(self.error_handler_spec.get(request.blueprint, {}).get(code))
-        
+        handler = find_handler(self.error_handler_spec
+                               .get(request.blueprint, {})
+                               .get(code))
         if handler is not None:
             return handler
-        
+
         # fall back to app handlers
-        return find_superclass(self.error_handler_spec[None].get(code))
+        return find_handler(self.error_handler_spec[None].get(code))
 
     def handle_http_exception(self, e):
         """Handles an HTTP exception.  By default this will invoke the
diff --git a/tests/test_user_error_handler.py b/tests/test_user_error_handler.py
index 78f4de3c..33131f3e 100644
--- a/tests/test_user_error_handler.py
+++ b/tests/test_user_error_handler.py
@@ -4,110 +4,108 @@ import flask
 
 
 def test_error_handler_subclass():
-	app = flask.Flask(__name__)
+    app = flask.Flask(__name__)
 
-	class ParentException(Exception):
-		pass
+    class ParentException(Exception):
+        pass
 
-	class ChildExceptionUnregistered(ParentException):
-		pass
+    class ChildExceptionUnregistered(ParentException):
+        pass
 
-	class ChildExceptionRegistered(ParentException):
-		pass
+    class ChildExceptionRegistered(ParentException):
+        pass
 
-	@app.errorhandler(ParentException)
-	def parent_exception_handler(e):
-		assert isinstance(e, ParentException)
-		return 'parent'
+    @app.errorhandler(ParentException)
+    def parent_exception_handler(e):
+        assert isinstance(e, ParentException)
+        return 'parent'
 
-	@app.errorhandler(ChildExceptionRegistered)
-	def child_exception_handler(e):
-		assert isinstance(e, ChildExceptionRegistered)
-		return 'child-registered'
+    @app.errorhandler(ChildExceptionRegistered)
+    def child_exception_handler(e):
+        assert isinstance(e, ChildExceptionRegistered)
+        return 'child-registered'
 
-	@app.route('/parent')
-	def parent_test():
-		raise ParentException()
+    @app.route('/parent')
+    def parent_test():
+        raise ParentException()
 
-	@app.route('/child-unregistered')
-	def unregistered_test():
-		raise ChildExceptionUnregistered()
+    @app.route('/child-unregistered')
+    def unregistered_test():
+        raise ChildExceptionUnregistered()
 
-	@app.route('/child-registered')
-	def registered_test():
-		raise ChildExceptionRegistered()
+    @app.route('/child-registered')
+    def registered_test():
+        raise ChildExceptionRegistered()
 
+    c = app.test_client()
 
-	c = app.test_client()
-
-	assert c.get('/parent').data == b'parent'
-	assert c.get('/child-unregistered').data == b'parent'
-	assert c.get('/child-registered').data == b'child-registered'
+    assert c.get('/parent').data == b'parent'
+    assert c.get('/child-unregistered').data == b'parent'
+    assert c.get('/child-registered').data == b'child-registered'
 
 
 def test_error_handler_http_subclass():
-	app = flask.Flask(__name__)
-
-	class ForbiddenSubclassRegistered(Forbidden):
-		pass
+    app = flask.Flask(__name__)
 
-	class ForbiddenSubclassUnregistered(Forbidden):
-		pass
+    class ForbiddenSubclassRegistered(Forbidden):
+        pass
 
-	@app.errorhandler(403)
-	def code_exception_handler(e):
-		assert isinstance(e, Forbidden)
-		return 'forbidden'
+    class ForbiddenSubclassUnregistered(Forbidden):
+        pass
 
-	@app.errorhandler(ForbiddenSubclassRegistered)
-	def subclass_exception_handler(e):
-		assert isinstance(e, ForbiddenSubclassRegistered)
-		return 'forbidden-registered'
+    @app.errorhandler(403)
+    def code_exception_handler(e):
+        assert isinstance(e, Forbidden)
+        return 'forbidden'
 
-	@app.route('/forbidden')
-	def forbidden_test():
-		raise Forbidden()
+    @app.errorhandler(ForbiddenSubclassRegistered)
+    def subclass_exception_handler(e):
+        assert isinstance(e, ForbiddenSubclassRegistered)
+        return 'forbidden-registered'
 
-	@app.route('/forbidden-registered')
-	def registered_test():
-		raise ForbiddenSubclassRegistered()
+    @app.route('/forbidden')
+    def forbidden_test():
+        raise Forbidden()
 
-	@app.route('/forbidden-unregistered')
-	def unregistered_test():
-		raise ForbiddenSubclassUnregistered()
+    @app.route('/forbidden-registered')
+    def registered_test():
+        raise ForbiddenSubclassRegistered()
 
+    @app.route('/forbidden-unregistered')
+    def unregistered_test():
+        raise ForbiddenSubclassUnregistered()
 
-	c = app.test_client()
+    c = app.test_client()
 
-	assert c.get('/forbidden').data == b'forbidden'
-	assert c.get('/forbidden-unregistered').data == b'forbidden'
-	assert c.get('/forbidden-registered').data == b'forbidden-registered'
+    assert c.get('/forbidden').data == b'forbidden'
+    assert c.get('/forbidden-unregistered').data == b'forbidden'
+    assert c.get('/forbidden-registered').data == b'forbidden-registered'
 
 
 def test_error_handler_blueprint():
-	bp = flask.Blueprint('bp', __name__)
-	
-	@bp.errorhandler(500)
-	def bp_exception_handler(e):
-		return 'bp-error'
-	
-	@bp.route('/error')
-	def bp_test():
-		raise InternalServerError()
-	
-	app = flask.Flask(__name__)
-	
-	@app.errorhandler(500)
-	def app_exception_handler(e):
-		return 'app-error'
-	
-	@app.route('/error')
-	def app_test():
-		raise InternalServerError()
-	
-	app.register_blueprint(bp, url_prefix='/bp')
-	
-	c = app.test_client()
-	
-	assert c.get('/error').data == b'app-error'
-	assert c.get('/bp/error').data == b'bp-error'
\ No newline at end of file
+    bp = flask.Blueprint('bp', __name__)
+
+    @bp.errorhandler(500)
+    def bp_exception_handler(e):
+        return 'bp-error'
+
+    @bp.route('/error')
+    def bp_test():
+        raise InternalServerError()
+
+    app = flask.Flask(__name__)
+
+    @app.errorhandler(500)
+    def app_exception_handler(e):
+        return 'app-error'
+
+    @app.route('/error')
+    def app_test():
+        raise InternalServerError()
+
+    app.register_blueprint(bp, url_prefix='/bp')
+
+    c = app.test_client()
+
+    assert c.get('/error').data == b'app-error'
+    assert c.get('/bp/error').data == b'bp-error'