diff --git a/muckrock/__init__.py b/muckrock/__init__.py
index 0d1182c..b0d9e08 100644
--- a/muckrock/__init__.py
+++ b/muckrock/__init__.py
@@ -228,7 +228,7 @@ def filter(
         if agency_id:
             assert isinstance(agency_id, int) or (
                 isinstance(agency_id, str) and agency_id.isdigit()
-            ), "agency_id must be a string"
+            ), "agency_id must be a integer"
             params["agency"] = agency_id
         datetime_submitted_choices = {
             None: 1,