Create resource conditionally with count and using filebase64sha256 function


I am not sure If I found a bug or that what I am doing here is wrong.
I am not that experienced yet using terraform.

We have setup a lambda resource like

resource "aws_lambda_layer_version" "lambda_requests_layer" {
  count   = var.some_var == "true" ? 1 : 0
  # Zip file is generated somewhere else and only available when the var above is true 
  filename            = ""
  layer_name          = "python_requests_layer"
  description         = "Python layer containing requests module"
  compatible_runtimes = ["python3.8"]
  source_code_hash   = filebase64sha256("")

However when running the terraform plan it complaints about the missing zip file even though the some_var variable is set to false and the resource should not be created.

error is like:
source_code_hash = filebase64sha256("./")
Call to function “filebase64sha256” failed: no file exists at

so it looks like the the filebase64sha256 function is triggered anyways.

Is there a way to prevent this… ?
Or is this some kind of a bug?

kind regards,