diff --git a/frontend/app/mstore/sessionStore.ts b/frontend/app/mstore/sessionStore.ts
index c9ff7c064..11a859d71 100644
--- a/frontend/app/mstore/sessionStore.ts
+++ b/frontend/app/mstore/sessionStore.ts
@@ -183,8 +183,13 @@ export default class SessionStore {
     }
     const nextEntryNum =
       keys.length > 0
-        ? Math.max(...keys.map((key) => this.prefetchedMobUrls[key].entryNum)) +
-          1
+        ? Math.max(
+            ...keys.map((key) =>
+              this.prefetchedMobUrls[key]
+                ? this.prefetchedMobUrls[key].entryNum
+                : 0
+            )
+          ) + 1
         : 0;
     this.prefetchedMobUrls[sessionId] = {
       data: fileData,