diff --git a/src/calendar/cached_calendar.rs b/src/calendar/cached_calendar.rs
index 22d9e63..f0c4f9e 100644
--- a/src/calendar/cached_calendar.rs
+++ b/src/calendar/cached_calendar.rs
@@ -97,8 +97,8 @@ impl PartialCalendar for CachedCalendar {
         Ok(map)
     }
 
-    async fn get_item_ids(&mut self) -> HashSet<ItemId> {
-        self.items.keys().cloned().collect()
+    async fn get_item_ids(&self) -> Result<HashSet<ItemId>, Box<dyn Error>> {
+        Ok(self.items.keys().cloned().collect())
     }
 
     async fn get_item_by_id_mut<'a>(&'a mut self, id: &ItemId) -> Option<&'a mut Item> {
diff --git a/src/provider.rs b/src/provider.rs
index 43402e0..b0e1cb6 100644
--- a/src/provider.rs
+++ b/src/provider.rs
@@ -83,7 +83,7 @@ where
                 Some(date) => cal_local.get_items_deleted_since(date).await?,
             };
             if last_sync.is_some() {
-                let server_deletions = cal_server.find_deletions_from(cal_local.get_item_ids().await).await;
+                let server_deletions = cal_server.find_deletions_from(cal_local.get_item_ids().await?).await?;
                 for server_del_id in server_deletions {
                     // Even in case of conflicts, "the server always wins", so it is safe to remove tasks from the local cache as soon as now
                     if let Err(err) = cal_local.delete_item(&server_del_id).await {
diff --git a/src/traits.rs b/src/traits.rs
index cfde874..34fe2d8 100644
--- a/src/traits.rs
+++ b/src/traits.rs
@@ -45,7 +45,7 @@ pub trait PartialCalendar {
         -> Result<HashMap<ItemId, &Item>, Box<dyn Error>>;
 
     /// Get the IDs of all current items in this calendar
-    async fn get_item_ids(&mut self) -> HashSet<ItemId>;
+    async fn get_item_ids(&self) -> Result<HashSet<ItemId>, Box<dyn Error>>;
 
     /// Returns a particular item
     async fn get_item_by_id_mut<'a>(&'a mut self, id: &ItemId) -> Option<&'a mut Item>;
@@ -68,9 +68,9 @@ pub trait PartialCalendar {
     }
 
     /// Finds the IDs of the items that are missing compared to a reference set
-    async fn find_deletions_from(&mut self, reference_set: HashSet<ItemId>) -> HashSet<ItemId> {
-        let current_items = self.get_item_ids().await;
-        reference_set.difference(&current_items).map(|id| id.clone()).collect()
+    async fn find_deletions_from(&self, reference_set: HashSet<ItemId>) -> Result<HashSet<ItemId>, Box<dyn Error>> {
+        let current_items = self.get_item_ids().await?;
+        Ok(reference_set.difference(&current_items).map(|id| id.clone()).collect())
     }
 }