diff --git a/tiingo/api.py b/tiingo/api.py
index 32b8be9..85cf58b 100644
--- a/tiingo/api.py
+++ b/tiingo/api.py
@@ -111,9 +111,9 @@ class TiingoClient(RestClient):
         if not len(assetTypes):
             return [row for row in reader]
 
-        t = set(assetTypes)
+        assetTypesSet = set(assetTypes)
         return [row for row in reader
-                if row.get('assetType') in t]
+                if row.get('assetType') in assetTypesSet]
 
     def list_stock_tickers(self):
         return self.list_tickers(['Stock'])