Terraform jsonencode() / yamlencode() Errors in CI

jsonencode()/yamlencode() serialize a Terraform value to JSON/YAML. They error when the value contains something they cannot represent, or when a referenced field is null/unknown at the point of encoding.

What this error means

plan fails inside a jsonencode()/yamlencode() call - often building an IAM policy document or a Kubernetes manifest - because a nested value is null, unknown, or an unsupported type.

Error: Error in function call

  on iam.tf line 4, in resource "aws_iam_role_policy" "app":
   4:   policy = jsonencode({ Statement = local.statements })

Call to function "jsonencode" failed: unsupported value: a null value cannot be
serialized as part of this JSON document.

Common causes

Null or unknown nested value

A field in the structure being encoded resolves to null (an unset optional) or is unknown until apply, which the encoder cannot serialize cleanly.

Unsupported value type

Passing a type the function cannot represent (for example a resource reference object instead of its concrete attributes) makes encoding fail.

How to fix it

Strip nulls and supply concrete values

Build the structure from concrete, non-null values; drop optional keys when their value is null.

locals {
  statement = merge(
    { Effect = "Allow", Action = var.actions, Resource = var.resource },
    var.condition == null ? {} : { Condition = var.condition }
  )
}
resource "aws_iam_role_policy" "app" {
  policy = jsonencode({ Version = "2012-10-17", Statement = [local.statement] })
}

Encode concrete attributes, not objects

1. Reference concrete attribute values (strings/numbers/lists), not whole resource objects.
2. Default optional variables so they are not null at encode time.
3. For apply-time-unknown values, ensure they are at least typed/known by the time the encode runs.

How to prevent it

• Default optional variables to avoid null fields in encoded structures.
• Omit map keys conditionally rather than setting them to null.
• Encode concrete attributes, not resource reference objects.

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