Aws waf ipset failes to create

I am trying to create a waf ipset with cloudflare ips in ( it keeps failing with /15 & /13 addresses,

Error: Error Updating WAF IPSet: Error Updating WAF IPSet: Error getting WAF change token: ValidationException: CIDR Prefix length 13 is invalid for IPV4.
status code: 400, request id: 5c5d6b8b-16b1-11ea-a979-9327190c4813

I can add them via the aws console.
Has anyone else had this issue?