diff --git a/docs/contributing/python_guidelines.rst b/docs/contributing/python_guidelines.rst
index 2428b53f9..7c75081bd 100644
--- a/docs/contributing/python_guidelines.rst
+++ b/docs/contributing/python_guidelines.rst
@@ -8,7 +8,7 @@ We ask that all Python contributions adhere to the `PEP8 <http://www.python.org/
 The list of PEP8 violations to ignore is in the ``tox.ini`` file, under the ``[flake8]`` header.
 You might want to configure the flake8 linter in your editor/IDE to use the configuration in this file.
 
-In addition, import lines should be sorted according to `isort <http://timothycrosley.github.io/isort/>`_ rules. If you have installed Wagtail's testing dependencies (``pip install -e .[testing]``), you can check your code by running ``make lint``.
+In addition, import lines should be sorted according to `isort <http://timothycrosley.github.io/isort/>`_ 4.2.5 rules. If you have installed Wagtail's testing dependencies (``pip install -e .[testing]``), you can check your code by running ``make lint``.
 
 
 Python 2 and 3 compatibility
diff --git a/setup.py b/setup.py
index 5cf3344a0..02da294c0 100755
--- a/setup.py
+++ b/setup.py
@@ -49,7 +49,7 @@ testing_extras = [
     # For coverage and PEP8 linting
     'coverage>=3.7.0',
     'flake8>=2.2.0',
-    'isort>=4.2.0',
+    'isort==4.2.5',
     'flake8-blind-except==0.1.1',
     'flake8-print==2.0.2',
 ]