diff --git a/parsons/etl/etl.py b/parsons/etl/etl.py
index 39df7a5044..e9647fb7a1 100644
--- a/parsons/etl/etl.py
+++ b/parsons/etl/etl.py
@@ -181,11 +181,16 @@ def convert_columns_to_str(self):
 
         cols = self.get_columns_type_stats()
 
+        def str_or_empty(x):
+            if x is None:
+                return ""
+            return str(x)
+
         for col in cols:
             # If there's more than one type (or no types), convert to str
             # Also if there is one type and it's not str, convert to str
             if len(col['type']) != 1 or col['type'][0] != 'str':
-                self.convert_column(col['name'], str)
+                self.convert_column(col['name'], str_or_empty)
 
         return self