Fixing this CIDR range for AWS VPC

Question

I am using CDK with C# to create a VPC and subnets in us-east-1.

    var newVpc = new Vpc(this, "myVPC", new VpcProps
    {
        MaxAzs = 3,
        IpAddresses = IpAddresses.Cidr("192.168.1.0/16"),
        EnableDnsHostnames = true,
        EnableDnsSupport = true,
    });

Here are the two private subnets I am attempting to create.

   var pvtSub1 = new Subnet(this, "pvt-sub1", new SubnetProps
   {
       VpcId = myVpc.VpcId,
       AvailabilityZone = "us-east-1a",
       CidrBlock = "192.168.1.0/28",
       MapPublicIpOnLaunch =false
   });

   var pvtSub2 = new Subnet(this, "pvt-sub2", new SubnetProps
   {
       VpcId = myVpc.VpcId,
       AvailabilityZone = "us-east-1b",
       CidrBlock = "192.168.1.16/28",
       MapPublicIpOnLaunch = false
   });

These are failing with error message:

Resource handler returned message: "The CIDR '192.168.1.0/28' is invalid
Resource handler returned message: "The CIDR '192.168.1.16/28' is invalid

Why would these be invalid? When I checked here the subnets 0 and 1 seem to be reflecting my specifications in CDK. So, what am I missing?