How to fix Error: Invalid template interpolation value in Terraform

Review your Terraform configuration files (.tf files) and look for the error message location. The error will point to a specific line and resource. Common patterns to look for include:

# Wrong - trying to interpolate a list directly
resource "example" "test" {
  config = "\${aws_subnet.example.*.id}"
}

# Wrong - using a null value in string
config = "\${var.optional_value}"

# Wrong - interpolating a map directly
output = "\${local.settings}"

Check the error message in your terraform output to see exactly which line and attribute has the problem.

If you're trying to interpolate a list or splat expression, use the join() function to convert it to a comma-separated string:

# Right - join the list into a string
resource "example" "test" {
  config = join(",", aws_subnet.example.*.id)
}

# Right - or if you need it in a template, use join inside
message = "Subnets: \${join(", ", aws_subnet.example.*.id)}"

The join() function takes a delimiter (first argument) and a list (second argument) and combines them into a single string.

If the error involves null values, use coalesce() to provide a default, or use a conditional expression:

# Right - provide a default if value is null
config = coalesce(var.optional_value, "default")

# Right - use conditional expression
config = var.optional_value != null ? var.optional_value : "default"

# Right - or simply omit the value if it's optional
optional_setting = var.optional_value

coalesce() returns the first non-null argument, making it useful for providing defaults.

If you're using Terraform 0.12 or newer, remove the ${} wrapper when a value is already an expression. This is a deprecated pattern from v0.11:

# Old style (Terraform 0.11) - now deprecated
instance_id = "\${aws_instance.example.id}"

# New style (Terraform 0.12+) - preferred
instance_id = aws_instance.example.id

In Terraform 0.12+, you can reference values directly without the interpolation syntax. Only use ${} when you actually need to embed an expression inside a string.

If you need to convert maps, objects, or other complex types to strings for templates, use jsonencode() or yamlencode():

# Right - convert map to JSON string
config = jsonencode(var.settings)

# Right - convert to YAML if needed
yaml_config = yamlencode(var.settings)

# Right - combine with template if needed
message = "Config: \${jsonencode(local.settings)}"

These functions handle the serialization of complex types safely.