diff --git a/requirements_dev.txt b/requirements_dev.txt
index b2f5109..2239f33 100644
--- a/requirements_dev.txt
+++ b/requirements_dev.txt
@@ -1,3 +1,4 @@
+six==1.14.0
 google-api-python-client>=1.7.4
 oauth2client>=4.1.2
 pytest
diff --git a/sheetfu/model.py b/sheetfu/model.py
index a077e26..bf041f0 100644
--- a/sheetfu/model.py
+++ b/sheetfu/model.py
@@ -696,3 +696,4 @@ def get_max_column(self):
         """
         return self.coordinates.column + self.coordinates.number_of_columns - 1
 
+