This Terraform error occurs when you provide an incompatible type (like a string, number, or object) where a list of strings is expected. Fix it by understanding Terraform's type system and using the correct list syntax in your configuration.
The "Unsuitable value type - list of string required" error is Terraform's way of telling you that an argument, variable, or attribute expects a list of strings, but you've provided something else. This is a type mismatch error. Terraform has a strict type system with specific expectations for each argument. When Terraform encounters a value that doesn't match the expected type and can't automatically convert it, it raises this error. This commonly happens with: - Passing a single string where a list is expected - Wrapping an already-list value in extra brackets - Using an incompatible data type (number, bool, map, etc.) where a list of strings is required - Configuration syntax errors or incorrect variable references
First, identify which argument or variable is causing the error. Check the error message and the resource documentation to understand what type it expects.
If the error says 'list of string required', that means the argument should be a list of strings like:
["value1", "value2", "value3"]
Check the provider documentation or resource schema for the specific argument that's failing.
Look at your configuration and check if you're wrapping a list in extra brackets.
WRONG (extra brackets):
instances = [var.instance_list]
CORRECT (use the list directly):
instances = var.instance_list
If you want to combine or manipulate lists, use list functions or the concat() function instead of wrapping.
In your variables.tf, ensure that any variable you're passing to a list-of-strings argument is actually defined as list(string).
Example of a properly defined list variable:
variable "allowed_users" {
type = list(string)
description = "List of allowed usernames"
}
If you have a single string that needs to become a list, either:
1. Define the variable as list(string) and wrap the value in square brackets when using it
2. Wrap the single value in a list at the point of use: allowed_users = [var.single_user]
If you have a single string value that needs to be used where a list of strings is expected, convert it to a list:
variable "single_user" {
type = string
}
resource "example_resource" "main" {
# Convert single string to a list of one string
users = [var.single_user]
}
Or use Terraform functions to manipulate the data:
resource "example_resource" "main" {
# Use tolist() to ensure it's a list
users = tolist([var.single_user])
}
If you need to apply a list of values to multiple resources or arguments, use for_each or count with proper list indexing:
variable "users" {
type = list(string)
}
resource "example_resource" "by_user" {
for_each = toset(var.users)
# Each iteration gets one string from the list
username = each.value
}
Run terraform validate to check for syntax errors before applying:
terraform validate
If you want to see what types Terraform has determined for your values, use terraform console:
terraform console
> var.my_variable
> type(var.my_variable)
This helps diagnose type mismatches quickly.
Type conversion in Terraform: Terraform will automatically convert compatible types when possible. For example, if an argument expects a list(string), Terraform can convert a tuple of mixed types like ["a", 15, true] to ["a", "15", "true"] by converting elements to strings.
However, this automatic conversion doesn't work for incompatible types. You cannot convert an object to a string or a map to a list without explicit conversion functions.
For Terraform v0.12+ users upgrading from v0.11: The error "list of string is required" often appears after upgrading because v0.12 removed support for redundant extra brackets around interpolations. Code that previously worked like instances = [var.instance_list] now needs to be instances = var.instance_list.
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