diff --git a/src/service/error.rs b/src/service/error.rs
index 74d89521..3323f1f3 100644
--- a/src/service/error.rs
+++ b/src/service/error.rs
@@ -157,14 +157,14 @@ impl<'a, T> SharedResultExtRef<'a, T> for &'a Result<T, SharedError> {
 
 #[macro_export]
 macro_rules! api_bail {
-    ( $fmt:literal $(, $($arg:expr) , *)?) => {
-        return Err($crate::service::error::ApiError::new(&format!($fmt $(, $($arg) , *)?), axum::http::StatusCode::BAD_REQUEST).into())
+    ( $fmt:literal $(, $($arg:tt)*)?) => {
+        return Err($crate::service::error::ApiError::new(&format!($fmt $(, $($arg)*)?), axum::http::StatusCode::BAD_REQUEST).into())
     };
 }
 
 #[macro_export]
 macro_rules! api_error {
-    ( $fmt:literal $(, $($arg:expr) , *)?) => {
-        $crate::service::error::ApiError::new(&format!($fmt $(, $($arg) , *)?), axum::http::StatusCode::BAD_REQUEST)
+    ( $fmt:literal $(, $($arg:tt)*)?) => {
+        $crate::service::error::ApiError::new(&format!($fmt $(, $($arg)*)?), axum::http::StatusCode::BAD_REQUEST)
     };
 }
diff --git a/src/setup/auth_registry.rs b/src/setup/auth_registry.rs
index fbe3769d..ee382fe7 100644
--- a/src/setup/auth_registry.rs
+++ b/src/setup/auth_registry.rs
@@ -36,7 +36,13 @@ impl AuthRegistry {
         let entries = self.entries.read().unwrap();
         match entries.get(&entry_ref.key) {
             Some(value) => Ok(serde_json::from_value(value.clone())?),
-            None => api_bail!("Auth entry not found: {}", entry_ref.key),
+            None => api_bail!(
+                "Auth entry `{key}` not found.\n\
+                Hint: If you're not referencing `{key}` in your flow, it will likely be caused by a previously persisted storage target using it. \
+                You need to bring back the definition for the auth entry `{key}`, so that CocoIndex will be able to do a cleanup in the next `setup` run. \
+                See https://cocoindex.io/docs/core/flow_def#auth-registry for more details.",
+                key = entry_ref.key
+            ),
         }
     }
 }