diff --git a/pystac_client/client.py b/pystac_client/client.py
index e843a5a..5621fc9 100644
--- a/pystac_client/client.py
+++ b/pystac_client/client.py
@@ -394,7 +394,7 @@ class Client(pystac.Catalog, QueryablesMixin):
         Raises:
             NotFoundError if collection_id does not exist.
         """
-        collection: Union[Collection, CollectionClient]
+        collection: Union[Collection, CollectionClient] = None
 
         if self._supports_collections():
             assert self._stac_io is not None
@@ -411,6 +411,9 @@ class Client(pystac.Catalog, QueryablesMixin):
             for collection in super().get_collections():
                 if collection.id == collection_id:
                     call_modifier(self.modifier, collection)
+                    break
+            if collection is None:
+                raise KeyError(f"Collection {collection_id} not found on catalog")
         return collection
 
     def get_collections(self) -> Iterator[Collection]: