E3012 Specify only 'SubnetMappings' or 'Subnets'

I am using Troposphere version 3.2.2 and Python 3.7.

I want to create 2 application load balancers and I am using albv2 template for the same. One load balancer is Internet facing and another is private hence I want to attach 3 public subnets (one for each AZ) to the internet facing load balancer and 3 private subnets (one for each AZ) to the private load balancer. I am doing that with ImportValue.

While doing that I am getting an error - E3012 Specify only 'SubnetMappings' or 'Subnets'. I am using "Subnets" at both places. Ideally this shouldn't have been an issue since the subnets are different and getting attached to different load balancers. Can someone guide please.

My code -

ABCALB = elbv2.LoadBalancer(
"ApplicationLoadBalancer",
Scheme="internet-facing",
SecurityGroups=[
If("IsTrue", Ref(AlbSecurityGroupPublic), Ref(AlbSecurityGroupPrivate))
],
Subnets=[
ImportValue(
Sub("${EnvironmentName}-ABC-vpc::PublicSubnet1a")
),
ImportValue(
Sub("${EnvironmentName}-ABC-vpc::PublicSubnet1b")
),
ImportValue(
Sub("${EnvironmentName}-ABC-vpc::PublicSubnet1c")
),
],
Tags=get_tags(
Sub("${EnvironmentName}-ABC-ALB"), disableDD=True
),
Type="application",
LoadBalancerAttributes=[
elbv2.LoadBalancerAttributes(Key="access_logs.s3.enabled", Value="true"),
elbv2.LoadBalancerAttributes(
Key="access_logs.s3.bucket",
Value=Sub("${EnvironmentName}-ABC-elb"),
),
elbv2.LoadBalancerAttributes(
Key="deletion_protection.enabled", Value="true"
),
elbv2.LoadBalancerAttributes(
Key="idle_timeout.timeout_seconds", Value="180"
),
],
)
template.add_resource(ABCALB)

ABCALBInternal = elbv2.LoadBalancer(
"ApplicationInternalLoadBalancer",
Scheme="internal",
SecurityGroups=[
If("IsTrue", Ref(AlbSecurityGroupPublic), Ref(AlbSecurityGroupPrivate))
],
Subnets=[
ImportValue(
Sub("${EnvironmentName}-ABC-vpc::PrivateSubnet1a")
),
ImportValue(
Sub("${EnvironmentName}-ABC-vpc::PrivateSubnet1b")
),
ImportValue(
Sub("${EnvironmentName}-ABC-vpc::PrivateSubnet1c")
),
],
Tags=get_tags(Sub("${EnvironmentName}ABC-internal-ALB")),
Type="application",
LoadBalancerAttributes=[
elbv2.LoadBalancerAttributes(Key="access_logs.s3.enabled", Value="true"),
elbv2.LoadBalancerAttributes(
Key="access_logs.s3.bucket",
Value=Sub("${EnvironmentName}-ABC-internal-elb"),
),
elbv2.LoadBalancerAttributes(
Key="deletion_protection.enabled", Value="true"
),
elbv2.LoadBalancerAttributes(
Key="idle_timeout.timeout_seconds", Value="180"
),
],
)
template.add_resource(ABCALBInternal)