Invalid IPv6 CIDR when creating Local Zone subnet

I'm trying to create a Local Zone subnet in us-east-1-dfw-2a, which supports IPv6, using a IPv6 CIDR range that fits in the /56 of its VPC, but I'm getting an invalid subnet range error.

Here's the event from Cloud Trail:

"errorCode": "Client.InvalidSubnet.Range",
"errorMessage": "The IPv6 CIDR '2600:1f18:48a8:REDACTED::/64' is invalid.",
"requestParameters": {
    "vpcId": "vpc-REDACTED",
    "cidrBlock": "10.2.0.112/28",
    "ipv6CidrBlock": "2600:1f18:48a8:REDACTED::/64",
    "availabilityZone": "us-east-1-dfw-2a",