diff --git a/src/matrix/room/timeline/FragmentIdComparer.js b/src/matrix/room/timeline/FragmentIdComparer.js
index 2c829418..fcd27edf 100644
--- a/src/matrix/room/timeline/FragmentIdComparer.js
+++ b/src/matrix/room/timeline/FragmentIdComparer.js
@@ -127,6 +127,10 @@ class Island {
     }
 }
 
+export class CompareError extends Error {
+    get name() { return "CompareError"; }
+}
+
 /*
 index for fast lookup of how two fragments can be sorted
 */
@@ -139,7 +143,7 @@ export class FragmentIdComparer {
     _getIsland(id) {
         const island = this._idToIsland.get(id);
         if (island === undefined) {
-            throw new Error(`Unknown fragment id ${id}`);
+            throw new CompareError(`Unknown fragment id ${id}`);
         }
         return island;
     }
@@ -151,7 +155,7 @@ export class FragmentIdComparer {
         const islandA = this._getIsland(idA);
         const islandB = this._getIsland(idB);
         if (islandA !== islandB) {
-            throw new Error(`${idA} and ${idB} are on different islands, can't tell order`);
+            throw new CompareError(`${idA} and ${idB} are on different islands, can't tell order`);
         }
         return islandA.compare(idA, idB);
     }
diff --git a/src/matrix/room/timeline/Timeline.js b/src/matrix/room/timeline/Timeline.js
index bd521e62..fdfaee48 100644
--- a/src/matrix/room/timeline/Timeline.js
+++ b/src/matrix/room/timeline/Timeline.js
@@ -212,7 +212,22 @@ export class Timeline {
     replaceEntries(entries) {
         this._addLocalRelationsToNewRemoteEntries(entries);
         for (const entry of entries) {
-            this._remoteEntries.getAndUpdate(entry, Timeline._entryUpdater);
+            try {
+                this._remoteEntries.getAndUpdate(entry, Timeline._entryUpdater);
+            } catch (err) {
+                if (err.name === "CompareError") {
+                    // see FragmentIdComparer, if the replacing entry is on a fragment
+                    // that is currently not loaded into the FragmentIdComparer, it will
+                    // throw a CompareError, and it means that the event is not loaded 
+                    // in the timeline (like when receiving a relation for an event
+                    // that is not loaded in memory) so we can just drop this error as
+                    // replacing an event that is not already loaded is a no-op.
+                    continue;
+                } else {
+                    // don't swallow other errors
+                    throw err;
+                }
+            }
         }
     }