How to fix Terraform error: Call to function “merge” failed: arguments must be maps or objects, got “tuple”?

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title: "Solving Terraform Error: Call to function \"merge\" failed: arguments must be maps or objects, got \"tuple\""
date: 2023-09-28
tags: terraform, terraform-aws-eks, aws

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![Terraform Error Troubleshooting](https://example.com/terraform-merge-error-header.jpg)  
*Learn why Terraform's merge function fails with "got tuple" error and how to fix it professionally.*

## Problem Summary
When merging AWS subnet tags using Terraform's `merge()` function, you encounter:

Error: Error in function call
Call to function “merge” failed: arguments must be maps or objects, got “tuple”.

This occurs when **passing incompatible data types** to Terraform's merge function, specifically wrapping a conditional expression in square brackets `[]` creates a tuple instead of a map. We'll explore why this happens and fix it properly.

## Root Cause
The core issue stems from **incorrect data types** passed to Terraform's `merge()` function:
```hcl
merge(
  { ... },  # Correct map
  [ ... ]   # WRONG: Wrapped in square brackets → Tuple!
)
  • merge() requires all arguments to be maps/objects
  • The ternary conditional var.is_public ? ... : ... already returns a map
  • Square brackets [ ] convert the map to a single-element tuple, causing type mismatch

Why This Happens in Real Systems

In practical Terraform modules:

  • Engineers often mix HCL collection types accidentally
  • Conditional logic in tags creates complexity
  • Copy-pasting code patterns from list contexts
  • Misunderstanding between:
    • {...} (map)
    • [...] (tuple/list)
    • (...) (grouping)

Real-World Impact

This bug blocks infrastructure deployment:

  • ❌ Prevents terraform apply execution
  • ❌ Halts CI/CD pipelines
  • ❌ Causes version conflicts when multiple engineers merge tags differently
  • ❌ Creates confusion between map vs. list operations

Code Example

Broken Implementation (Causes Error)

# SUBET MODULE (modules/subnet/main.tf)
tags = merge(
  {
    Name = var.name
    "kubernetes.io/cluster/${var.cluster_name}" = "shared"
  },
  [var.is_public ? { "kubernetes.io/role/elb" = 1 } : { "kubernetes.io/role/internal-elb" = 1 }]
  # ^-- Square brackets create TUPLE!
)

Fixed Implementation

# CORRECTED SUBET MODULE (modules/subnet/main.tf)
tags = merge(
  {
    Name = var.name
    "kubernetes.io/cluster/${var.cluster_name}" = "shared"
  },
  var.is_public ? { "kubernetes.io/role/elb" = 1 } : { "kubernetes.io/role/internal-elb" = 1 }
  # No brackets → Direct map output
)

How Senior Engineers Fix It

  1. Remove the tuple wrapper: Eliminate [ ] around ternary expressions
  2. Validate types: Use type() in console to debug: 
    terraform console > type(var.is_public ? {a=1} : {b=2})
object
  3. Use merge() alternatives for complex cases: 
    tags = { 
  for k,v in merge(
    local.base_tags,
    var.extra_tags
  ) : k => v
}
  4. Implement pre-merge validation: 
    validation {
  condition     = can(tomap(var.conditional_tags))
  error_message = "Conditional tags must return map"
}

Why Juniors Miss It

Junior engineers often struggle here because:

  • Confusing HCL syntax: Square brackets [] vs curly braces {}
  • Misunderstanding ternary outputs: Not realizing ? : returns objects directly
  • Limited type checking: Forgetting to validate variable structures
  • Error message tunnel vision: Focusing on “tuple” without debugging the input
  • Copy-paste habits: Reusing list patterns in map contexts

Pro Tip: Always use terraform console to validate expression outputs before merging them!

Fix this error today and deploy your subnet tags with confidence. 🚀
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