How to Fix CalDavSynchronizer Error: 'Mapping for value -1 not implemented' (Outlook + Be Human Calendar)

If you use CalDavSynchronizer to sync Microsoft Outlook with a CalDAV calendar (such as Be Human Calendar) and you get a synchronization error like this:

System.NotImplementedException: Mapping for value '-1' not implemented. at CalDavSynchronizer.Implementation.Events.EventEntityMapper.MapTransparency1To2

…this guide will fix it in just a couple of minutes.

What Causes the Error?

One single appointment in your Outlook calendar has a corrupted or invalid "Show As" (BusyStatus) value of -1. CalDavSynchronizer doesn't know how to handle this value when creating or updating the event on the server, so the entire sync aborts.

Step-by-Step Fix

Step 1: Open the Problematic Appointment Directly

1. In Outlook, press Alt + F11 to open the VBA editor.
2. Press Ctrl + G to open the Immediate Window.
3. Copy and paste the long ID from your Synchronization Report (it looks like this):

Application.Session.GetItemFromID("00000000FC7FF7F06275CC43B2948D70D3E4A3B00700C2647EB22E8F964DB6C66F29CD7258EE00000000010D0000C2647EB22E8F964DB6C66F29CD7258EE000782854BF10000").Display

4. Press Enter. Outlook will instantly open the guilty appointment.

Step 2: Fix or Delete the Event

You have two easy options:

• Recommended if the event is old or no longer needed: Just click Delete.
• If you want to keep the event: Change the Show As dropdown (top toolbar) from blank/None to Free, Busy, or Tentative, then click Save & Close.

Step 3: Resync

1. Open CalDavSynchronizer.
2. Click Synchronize Now.
3. The sync should now complete successfully with no errors.

Alternative Ways to Find Bad Events (Without VBA)

• Add the "Show As" column to your Outlook Calendar view.
• Sort by the Show As column.
• Look for any appointments showing as blank or "None".

Why This Works

The error only affects one event. Once that event is removed or corrected, normal syncing resumes.

Prevention Tips

• Keep CalDavSynchronizer updated.
• Be cautious when copying old appointments or importing calendars from other programs.
• Regularly review very old events in your calendar.

Problem solved. This exact method has resolved the "-1" mapping error for many users syncing Outlook with CalDAV services.