diff --git a/doc/install.rst b/doc/install.rst
index ae79f49f..2e63a6a3 100644
--- a/doc/install.rst
+++ b/doc/install.rst
@@ -12,6 +12,7 @@ pyOpt has the following dependencies:
 * Swig 1.3+
 * c/FORTRAN compiler (compatible with f2py)
 * mpi4py
+* six 1.13
 
 Notes:
 
diff --git a/pyoptsparse/pyOpt_optimization.py b/pyoptsparse/pyOpt_optimization.py
index fe231e7e..2c800407 100644
--- a/pyoptsparse/pyOpt_optimization.py
+++ b/pyoptsparse/pyOpt_optimization.py
@@ -30,16 +30,15 @@
 except ImportError:
     try:
         from ordereddict import OrderedDict
-    except ImportError:
-        print('Could not find any OrderedDict class. For 2.6 and earlier, \
+    except:
+        raise ImportError('Could not find any OrderedDict class. For 2.6 and earlier, \
 use:\n pip install ordereddict')
 
 try:
     import six
     from six import iteritems, iterkeys, next
-except ImportError:
-    six = None
-    print ('Could not import \'six\' OpenMDAO type tuple return not available.')
+except:
+    raise ImportError('Could not import \'six\'. To install, use\n pip install six')
 
 from .sqlitedict.sqlitedict import SqliteDict
 
@@ -1325,12 +1324,8 @@ def processObjectiveGradient(self, funcsSens):
         gobj = numpy.zeros((nobj, self.ndvs))
 
         cond = False
-        if six:
-            # this version is required for python 3 compatibility
-            cond = isinstance(next(iterkeys(funcsSens)), str)
-        else:
-            # fallback if six is not available
-            cond = isinstance(funcsSens.keys()[0], str)
+        # this version is required for python 3 compatibility
+        cond = isinstance(next(iterkeys(funcsSens)), str)
 
         if cond:
             iObj = 0