diff --git a/islpy/__init__.py b/islpy/__init__.py
index bbfef9bd5c9463f6eb074aebbd86b8958975e787..2c0a53d68f1f6ac763558c0470fea68d359c7f5a 100644
--- a/islpy/__init__.py
+++ b/islpy/__init__.py
@@ -556,17 +556,9 @@ def _add_functionality():
         if context is None:
             context = _DEFAULT_CONTEXT
 
-        import sys
-        if sys.version_info >= (3,):
-            string_types = str
-            int_types = int
-        else:
-            string_types = six.string_types
-            int_types = six.integer_types
-
-        if isinstance(src, string_types):
+        if isinstance(src, six.string_types):
             result = cls.read_from_str(context, src)
-        elif isinstance(src, int_types):
+        elif isinstance(src, six.integer_types):
             result = cls.int_from_si(context, src)
         else:
             raise TypeError("'src' must be int or string")
diff --git a/setup.py b/setup.py
index 635b6e9efeb5aa2b023280e269f7200fa6ee18dd..7054e5fba5122d3b534e5e308f25a4f942403c31 100644
--- a/setup.py
+++ b/setup.py
@@ -146,6 +146,7 @@ def main():
           install_requires=[
               "pytest>=2",
               # "Mako>=0.3.6",
+              "six",
               ],
           ext_modules=[
               Extension(