Terraform always update Lambda Layers version

[doi-t]

This happens all the time even there is no change in layer's source code.

-/+ aws_lambda_layer_version.event_to_slack_lambda_layer (new resource required)
      id:                             "arn:aws:lambda:ap-northeast-1:042234535815:layer:event-to-slack-lambda-layer:63" => <computed> (forces new resource)
      arn:                            "arn:aws:lambda:ap-northeast-1:042234535815:layer:event-to-slack-lambda-layer" => <computed>
      compatible_runtimes.#:          "1" => "1"
      compatible_runtimes.4150330526: "python3.7" => "python3.7"
      created_date:                   "2019-02-02T11:11:32.492+0000" => <computed>
      filename:                       ".terraform/modules/5e3d6e14e0a846a97689b73442597ced/packages/event-to-slack-lambda-layer.zip" => ".terraform/modules/5e3d6e14e0a846a97689b73442597ced/packages/event-to-slack-lambda-layer.zip"
      layer_arn:                      "arn:aws:lambda:ap-northeast-1:042234535815:layer:event-to-slack-lambda-layer:63" => <computed>
      layer_name:                     "event-to-slack-lambda-layer" => "event-to-slack-lambda-layer"
      source_code_hash:               "Gq1u0O1Tnv8pAi6Us/bGNnDVftBgNZbV/C4PVTybRUA=" => "${module.layer_package.zip_package_sha256}" (forces new resource)
      source_code_size:               "8076392" => <computed>
      version:                        "63" => <computed>

This might be because the module uses local zip file instead of S3 (?).