diff --git a/tests/unit/oidc/test_views.py b/tests/unit/oidc/test_views.py
index e0cabe4a2702..c6b17ee5b68d 100644
--- a/tests/unit/oidc/test_views.py
+++ b/tests/unit/oidc/test_views.py
@@ -246,6 +246,12 @@ def body(self):
         assert isinstance(err, dict)
         assert err["code"] == "invalid-payload"
         assert err["description"] == "unknown trusted publishing issuer"
+    assert metrics.increment.calls == [
+        pretend.call(
+            "warehouse.oidc.mint_token_from_oidc.unknown_issuer",
+            tags=["issuer_url:nonexistent-issuer"],
+        )
+    ]
 
 
 @pytest.mark.parametrize(
diff --git a/warehouse/oidc/views.py b/warehouse/oidc/views.py
index a9663b95ff21..261def5da8f8 100644
--- a/warehouse/oidc/views.py
+++ b/warehouse/oidc/views.py
@@ -147,7 +147,7 @@ def mint_token_from_oidc(request: Request):
     if not service_name:
         request.metrics.increment(
             "warehouse.oidc.mint_token_from_oidc.unknown_issuer",
-            tags={"issuer_url": unverified_issuer},
+            tags=[f"issuer_url:{unverified_issuer}"],
         )
         return _invalid(
             errors=[