diff --git a/synthpop/census_helpers.py b/synthpop/census_helpers.py
index 3516cfc..78331ef 100644
--- a/synthpop/census_helpers.py
+++ b/synthpop/census_helpers.py
@@ -173,7 +173,9 @@ def _read_csv(self, loc, **kargs):
             pums_df = pd.read_csv(loc, dtype={
                 "PUMA10": "object",
                 "PUMA00": "object",
-                "ST": "object"
+                "ST": "object",
+                "SERIALNO": 'str',
+                "serialno": 'str',
             }, **kargs)
             pums_df = pums_df.rename(columns={
                 'PUMA10': 'puma10',