diff --git a/src/matrix/room/timeline/FragmentIdComparer.js b/src/matrix/room/timeline/FragmentIdComparer.js
index 15c2bc08..c26aa043 100644
--- a/src/matrix/room/timeline/FragmentIdComparer.js
+++ b/src/matrix/room/timeline/FragmentIdComparer.js
@@ -22,31 +22,31 @@ until no more fragments
 
 */
 
+import {isValidFragmentId} from "./common.js";
 
 function findBackwardSiblingFragments(current, byId) {
     const sortedSiblings = [];
-    while (current.previousId) {
+    while (isValidFragmentId(current.previousId)) {
         const previous = byId.get(current.previousId);
         if (!previous) {
-            throw new Error(`Unknown previousId ${current.previousId} on ${current.id}`);
+            break;
         }
         if (previous.nextId !== current.id) {
             throw new Error(`Previous fragment ${previous.id} doesn't point back to ${current.id}`);
         }
         byId.delete(current.previousId);
-        sortedSiblings.push(previous);
+        sortedSiblings.unshift(previous);
         current = previous;
     }
-    sortedSiblings.reverse();
     return sortedSiblings;
 }
 
 function findForwardSiblingFragments(current, byId) {
     const sortedSiblings = [];
-    while (current.nextId) {
+    while (isValidFragmentId(current.nextId)) {
         const next = byId.get(current.nextId);
         if (!next) {
-            throw new Error(`Unknown nextId ${current.nextId} on ${current.id}`);
+            break;
         }
         if (next.previousId !== current.id) {
             throw new Error(`Next fragment ${next.id} doesn't point back to ${current.id}`);
@@ -143,7 +143,7 @@ export default class FragmentIdComparer {
     }
 
     add(fragment) {
-        this._fragmentsById[fragment.id] = fragment;
+        this._fragmentsById.set(fragment.id, fragment);
         this.rebuild(this._fragmentsById.values());
     }
 }
@@ -168,6 +168,44 @@ export function tests() {
             
             assert.equal(index.compare(1, 1), 0);
         },
+        test_falsy_id(assert) {
+            const index = new FragmentIdComparer([
+                {id: 0, nextId: 1},
+                {id: 1, previousId: 0},
+            ]);
+            assert(index.compare(0, 1) < 0);
+            assert(index.compare(1, 0) > 0);
+        },
+        test_falsy_id_reverse(assert) {
+            const index = new FragmentIdComparer([
+                {id: 1, previousId: 0},
+                {id: 0, nextId: 1},
+            ]);
+            assert(index.compare(0, 1) < 0);
+            assert(index.compare(1, 0) > 0);
+        },
+        test_allow_unknown_id(assert) {
+            // as we tend to load fragments incrementally
+            // as events come into view, we need to allow
+            // unknown previousId/nextId in the fragments that we do load
+            assert.doesNotThrow(() => {
+                new FragmentIdComparer([
+                    {id: 1, previousId: 2},
+                    {id: 0, nextId: 3},
+                ]);
+            });
+        },
+        test_throw_on_link_mismatch(assert) {
+            // as we tend to load fragments incrementally
+            // as events come into view, we need to allow
+            // unknown previousId/nextId in the fragments that we do load
+            assert.throws(() => {
+                new FragmentIdComparer([
+                    {id: 1, previousId: 0},
+                    {id: 0, nextId: 2},
+                ]);
+            });
+        },
         test_2_island_dont_compare(assert) {
             const index = new FragmentIdComparer([
                 {id: 1},
diff --git a/src/matrix/room/timeline/common.js b/src/matrix/room/timeline/common.js
new file mode 100644
index 00000000..7565d8a4
--- /dev/null
+++ b/src/matrix/room/timeline/common.js
@@ -0,0 +1,3 @@
+export function isValidFragmentId(id) {
+    return typeof id === "number";
+}
diff --git a/src/matrix/room/timeline/entries/FragmentBoundaryEntry.js b/src/matrix/room/timeline/entries/FragmentBoundaryEntry.js
index ce496364..3182fd2f 100644
--- a/src/matrix/room/timeline/entries/FragmentBoundaryEntry.js
+++ b/src/matrix/room/timeline/entries/FragmentBoundaryEntry.js
@@ -1,5 +1,6 @@
 import BaseEntry from "./BaseEntry.js";
 import Direction from "../Direction.js";
+import {isValidFragmentId} from "../common.js";
 
 export default class FragmentBoundaryEntry extends BaseEntry {
     constructor(fragment, isFragmentStart, fragmentIdComparator) {
@@ -77,6 +78,10 @@ export default class FragmentBoundaryEntry extends BaseEntry {
         }
     }
 
+    get hasLinkedFragment() {
+        return isValidFragmentId(this.linkedFragmentId);
+    }
+
     get direction() {
         if (this.started) {
             return Direction.Backward;
diff --git a/src/matrix/room/timeline/persistence/TimelineReader.js b/src/matrix/room/timeline/persistence/TimelineReader.js
index dd57ca39..7d86115b 100644
--- a/src/matrix/room/timeline/persistence/TimelineReader.js
+++ b/src/matrix/room/timeline/persistence/TimelineReader.js
@@ -25,7 +25,6 @@ export default class TimelineReader {
 
     async _readFrom(eventKey, direction, amount, txn) {
         let entries = [];
-
         const timelineStore = txn.timelineEvents;
         const fragmentStore = txn.timelineFragments;
         
@@ -47,7 +46,7 @@ export default class TimelineReader {
                 // append or prepend fragmentEntry, reuse func from GapWriter?
                 directionalAppend(entries, fragmentEntry, direction);
                 // don't count it in amount perhaps? or do?
-                if (fragmentEntry.linkedFragmentId) {
+                if (fragmentEntry.hasLinkedFragment) {
                     const nextFragment = await fragmentStore.get(this._roomId, fragmentEntry.linkedFragmentId);
                     this._fragmentIdComparer.add(nextFragment);
                     const nextFragmentEntry = new FragmentBoundaryEntry(nextFragment, direction.isForward, this._fragmentIdComparer);