diff --git a/.travis.yml b/.travis.yml
index 8d1f6a89..d22b6145 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,6 +15,8 @@ matrix:
         env: TOX_ENV=py26
       - python: 2.7
         env: TOX_ENV=py27
+      - python: 2.7
+        env: TOX_ENV=py27_old_six
       - python: 3.3
         env: TOX_ENV=py33
       - python: 3.4
diff --git a/setup.py b/setup.py
index c0959d44..13c91646 100755
--- a/setup.py
+++ b/setup.py
@@ -40,5 +40,5 @@
           "Programming Language :: Python :: 3.7",
           "Programming Language :: Python :: 3.8",
       ],
-      install_requires=['six'],
+      install_requires=['six>=1.9.0'],
       )
diff --git a/tox.ini b/tox.ini
index fbaf823e..0b3bcdad 100644
--- a/tox.ini
+++ b/tox.ini
@@ -12,12 +12,19 @@ deps =
      py{26}: hypothesis<3
      py{26,27,34,35,36,37,38,py,py3}: pytest
      py{27,34,35,36,37,38,py,py3}: hypothesis
+# six==1.9.0 comes from setup.py install_requires
+     py27_old_six: six==1.9.0
+     py27_old_six: pytest
+     py27_old_six: hypothesis
      py: pytest
      py: hypothesis
      py{33}: wheel<0.30
      coverage
 commands = coverage run --branch -m pytest {posargs:src/ecdsa}
 
+[testenv:py27_old_six]
+basepython = python2.7
+
 [testenv:coverage]
 sitepackages=True
 commands = coverage run --branch -m pytest --hypothesis-show-statistics {posargs:src/ecdsa}