diff --git a/jmclient/jmclient/blockchaininterface.py b/jmclient/jmclient/blockchaininterface.py
index 9dd58d015..3a646c919 100644
--- a/jmclient/jmclient/blockchaininterface.py
+++ b/jmclient/jmclient/blockchaininterface.py
@@ -590,7 +590,7 @@ def get_best_block_hash(self) -> str:
     def get_best_block_median_time(self) -> int:
         return self._rpc('getblockchaininfo', [])['mediantime']
 
-    def _get_block_header_data(self, blockhash: str, key: str):
+    def _get_block_header_data(self, blockhash: str, key: str) -> Any:
         return self._rpc('getblockheader', [blockhash])[key]
 
     def get_block_height(self, blockhash: str) -> int: