diff --git a/lib/charms/observability_libs/v0/cert_handler.py b/lib/charms/observability_libs/v0/cert_handler.py
index 24ec5c0..7494901 100644
--- a/lib/charms/observability_libs/v0/cert_handler.py
+++ b/lib/charms/observability_libs/v0/cert_handler.py
@@ -382,7 +382,7 @@ def _chain(self) -> str:
         return ""
 
     @_chain.setter
-    def _chain(self, value: List[str]):
+    def _chain(self, value: str):
         # Caller must guard. We want the setter to fail loudly. Failure must have a side effect.
         rel = self._peer_relation
         assert rel is not None  # For type checker