It's possible (for now) for a Ritual to not have a fee model address associated with it (legacy lynx for example).

nucypher/blockchain/eth/agents.py

@@ -593,6 +593,8 @@ class CoordinatorAgent(EthereumContractAgent):
     @contract_api(CONTRACT_CALL)
     def __rituals(self, ritual_id: int) -> Coordinator.Ritual:
         result = self.contract.functions.rituals(int(ritual_id)).call()
+        # legacy rituals do not have a fee model entry - so None is appropriate
+        fee_model = ChecksumAddress(result[12]) if len(result) >= 13 else None
         ritual = Coordinator.Ritual(
             id=ritual_id,
             initiator=ChecksumAddress(result[0]),
@@ -608,7 +610,7 @@ class CoordinatorAgent(EthereumContractAgent):
             public_key=Ferveo.G1Point(result[10][0], result[10][1]),
             aggregated_transcript=bytes(result[11]),
             participants=[],  # solidity does not return sub-structs
-            fee_model=ChecksumAddress(result[12]),
+            fee_model=fee_model,
         )
         return ritual
 

nucypher/blockchain/eth/models.py

@@ -117,11 +117,11 @@ class Coordinator:
         initiator: ChecksumAddress
         authority: ChecksumAddress
         access_controller: ChecksumAddress
-        fee_model: ChecksumAddress
         dkg_size: int
         init_timestamp: int
         end_timestamp: int
         threshold: int
+        fee_model: ChecksumAddress = None
         total_transcripts: int = 0
         total_aggregations: int = 0
         public_key: Ferveo.G1Point = None