diff --git a/readthedocs/doc_builder/python_environments.py b/readthedocs/doc_builder/python_environments.py
index c6fa5590352..82273c8235e 100644
--- a/readthedocs/doc_builder/python_environments.py
+++ b/readthedocs/doc_builder/python_environments.py
@@ -367,6 +367,7 @@ def install_core_requirements(self):
             'mock==1.0.1',
             'pillow==5.4.1',
             'alabaster>=0.7,<0.8,!=0.7.5',
+            'six',
             'commonmark==0.8.1',
             'recommonmark==0.5.0',
         ]
@@ -623,6 +624,7 @@ def _get_core_requirements(self):
         # Install pip-only things.
         pip_requirements = [
             'recommonmark',
+            'six',
         ]
 
         if self.config.doctype == 'mkdocs':
diff --git a/readthedocs/rtd_tests/tests/test_doc_building.py b/readthedocs/rtd_tests/tests/test_doc_building.py
index 5fc655a1f9b..15efe9b89d1 100644
--- a/readthedocs/rtd_tests/tests/test_doc_building.py
+++ b/readthedocs/rtd_tests/tests/test_doc_building.py
@@ -1197,6 +1197,7 @@ def setUp(self):
             'mock',
             'pillow',
             'alabaster',
+            'six',
         ]
         self.base_conda_requirements = [
             'mock',
@@ -1364,6 +1365,7 @@ def test_install_core_requirements_sphinx_conda(self, checkout_path):
         conda_requirements = self.base_conda_requirements + conda_sphinx
         pip_requirements = [
             'recommonmark',
+            'six',
             'readthedocs-sphinx-ext',
         ]
 
@@ -1405,6 +1407,7 @@ def test_install_core_requirements_mkdocs_conda(self, checkout_path):
         conda_requirements = self.base_conda_requirements
         pip_requirements = [
             'recommonmark',
+            'six',
             'mkdocs',
         ]