Couchbase server 4.0 cbbackup not working

While running cbbackup on a single node couchbase server using couchstore-files, I’m getting the following error:
error: could not read couchstore file due to unsupported file format version; exception: unpack requires a string argument of length 19

I recently upgraded the node from v3.0.2 to v4.0.0 after which I’m seeing this error. I guess cbbackup is not understanding the file format of 3.0.2 data buckets. Any ideas?

edit: The same error occurs on 3 different single node clusters recently upgraded from v3.0.2. This doesn’t seem to be a node/server specific issue