This error occurs when a Terraform function receives an argument with an incompatible type or incorrect number of parameters. Common causes include type mismatches, missing files, or using the wrong function syntax.
The 'Invalid function argument' error indicates that the function call in your Terraform configuration is receiving an argument that doesn't match the function's expected input types or structure. Each Terraform built-in function has specific requirements for the types and number of arguments it accepts. When you provide arguments that don't meet these requirements, Terraform validation fails before deployment.
Check the Terraform documentation at https://developer.hashicorp.com/terraform/language/functions to understand the exact argument requirements for the function causing the error. Note the expected argument types, count, and any constraints.
Use terraform console to inspect the types of values you're passing:
terraform console
# Then enter: type(your_variable)
# Or: your_function_callThis helps verify that your arguments match the expected types.
If using file(), filebase64(), or similar functions:
# Incorrect: relative path that may not exist
data = file("config.json")
# Correct: absolute path or path relative to module root
data = file("${path.module}/config.json")File functions only work with files distributed as part of your configuration, not files created by resources. Use resource attributes instead:
# Wrong: file() on output of a resource
config = file(aws_s3_object.config.key)
# Right: use the resource's output attribute
config = aws_s3_object.config.bodyFor functions like join(), concat(), lookup():
# Incorrect: mixing types
result = join("-", var.names) # if var.names contains numbers
# Correct: convert to strings first
result = join("-", [for name in var.names : tostring(name)])For lookup() with defaults:
# Incorrect: type mismatch
value = lookup(local.config, "key", 0) # map values are strings, default is number
# Correct: matching types
value = lookup(local.config, "key", "default")When passing a list as individual arguments, use the ... operator:
# Incorrect: passing list directly
subnets = cidrsubnets("10.1.0.0/16", local.new_bits)
# Correct: spreading the list
subnets = cidrsubnets("10.1.0.0/16", local.new_bits...)This expands list elements as separate arguments.
After making corrections, validate the syntax:
terraform validateIf validation passes, proceed with:
terraform plan
terraform applyType mismatches often occur during Terraform version upgrades. Terraform 0.12 introduced strict type checking that wasn't present in 0.11. If upgrading, carefully review all function calls in your configuration. For dynamic scenarios where argument types are computed at runtime, use try() or conditional logic to handle different types gracefully. When working with modules, ensure outputs from child modules have the correct types expected by parent module functions.
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