diff --git a/client/src/components/InvitationAcceptance.tsx b/client/src/components/InvitationAcceptance.tsx
index a0c5063..399059a 100644
--- a/client/src/components/InvitationAcceptance.tsx
+++ b/client/src/components/InvitationAcceptance.tsx
@@ -10,6 +10,7 @@ import {
 } from '../services/auth.js'
 import { decryptJson, encryptBuffer } from '../services/crypto.js'
 import { saveLogbookKey } from '../services/logbookKeys.js'
+import { parseCollaborationRole } from '../services/logbook.js'
 import { syncLogbook } from '../services/sync.js'
 import { db } from '../services/db.js'
 import { PlausibleEvents, trackPlausibleEvent } from '../services/analytics.js'
@@ -183,6 +184,7 @@ export default function InvitationAcceptance({ onAccepted, onCancel }: Invitatio
       }
 
       const acceptResult = await res.json()
+      const collaborationRole = parseCollaborationRole(acceptResult.role, 'invitation accept')
 
       await saveLogbookKey(logbookId, logbookKey)
 
@@ -193,7 +195,7 @@ export default function InvitationAcceptance({ onAccepted, onCancel }: Invitatio
           updatedAt: new Date().toISOString(),
           isSynced: 1,
           isShared: 1,
-          collaborationRole: acceptResult.role === 'READ' ? 'READ' : 'WRITE'
+          collaborationRole
         })
       }
 
diff --git a/client/src/services/logbook.ts b/client/src/services/logbook.ts
index 3ec80fe..3db0607 100644
--- a/client/src/services/logbook.ts
+++ b/client/src/services/logbook.ts
@@ -7,6 +7,22 @@ import { PlausibleEvents, trackPlausibleEvent } from './analytics.js'
 const API_BASE = '/api/logbooks'
 
 export type LogbookAccessRole = 'OWNER' | 'READ' | 'WRITE'
+export type CollaborationRole = 'READ' | 'WRITE'
+
+/** Validates server/cached collaboration role; warns and falls back to WRITE if missing or invalid. */
+export function parseCollaborationRole(role: unknown, context: string): CollaborationRole {
+  if (role === 'READ' || role === 'WRITE') {
+    return role
+  }
+
+  if (role === undefined || role === null || role === '') {
+    console.warn(`[collaboration] Missing role in ${context}; defaulting to WRITE.`)
+  } else {
+    console.warn(`[collaboration] Unexpected role in ${context}:`, role, '— defaulting to WRITE.')
+  }
+
+  return 'WRITE'
+}
 
 export interface DecryptedLogbook {
   id: string
@@ -112,7 +128,9 @@ export async function fetchLogbooks(): Promise<DecryptedLogbook[]> {
             updatedAt: lb.updatedAt || new Date().toISOString(),
             isSynced: 1,
             isShared: isShared ? 1 : 0,
-            collaborationRole: isShared ? (lb.collaborators?.[0]?.role || 'WRITE') : undefined,
+            collaborationRole: isShared
+              ? parseCollaborationRole(lb.collaborators?.[0]?.role, `fetch logbook ${lb.id}`)
+              : undefined,
             isDemo: localById.get(lb.id)?.isDemo
           }
         })
@@ -138,7 +156,9 @@ export async function fetchLogbooks(): Promise<DecryptedLogbook[]> {
       updatedAt: lb.updatedAt,
       isSynced: lb.isSynced === 1,
       isShared: lb.isShared === 1,
-      accessRole: lb.isShared === 1 ? (lb.collaborationRole || 'WRITE') : 'OWNER',
+      accessRole: lb.isShared === 1
+        ? parseCollaborationRole(lb.collaborationRole, `cached logbook ${lb.id}`)
+        : 'OWNER',
       isDemo: lb.isDemo === 1
     })
   }