diff --git a/ib_insync/ib.py b/ib_insync/ib.py
index 96012bca..c18d9f35 100644
--- a/ib_insync/ib.py
+++ b/ib_insync/ib.py
@@ -1857,6 +1857,7 @@ async def qualifyContractsAsync(self, *contracts: Contract) \
                     f'possibles are {possibles}')
             else:
                 c = detailsList[0].contract
+                assert c
                 if contract.exchange == 'SMART':
                     # overwriting 'SMART' exchange can create invalid contract
                     c.exchange = contract.exchange