You did a zappa deploy and it failed with InvalidParameterValueException: An error occurred (InvalidParameterValueException) when calling the CreateFunction operation: The role defined for the function cannot be assumed by Lambda?

You tried to create a lambda with a new handmade role only to be greeted by this cryptic error message. Or you tried to use an already existing role with lambda.

Translating the message: it means you haven't authorized the lambda service to assume the role, so lambdas can't use it. So, how do we add that authorization?

• Go to https://console.aws.amazon.com/iam/
• Click roles on the left.
• Click the role you want to use for lambda.
• Click the tab trust relationships.
• Click the button Edit trust relationship.
• If this lambda is only to be used by lambda, you can just replace the policy by:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "",
            "Effect": "Allow",
            "Principal": {
                "Service": [
                    "apigateway.amazonaws.com",
                    "lambda.amazonaws.com",
                    "events.amazonaws.com"
                ]
            },
            "Action": "sts:AssumeRole"
        }
    ]
}

• If not, just make sure you add to the Statement list the statement:

        {
          "Sid": "",
          "Effect": "Allow",
          "Principal": {
            "Service": [
              "apigateway.amazonaws.com",
              "lambda.amazonaws.com",
              "events.amazonaws.com"
            ]
          },
          "Action": "sts:AssumeRole"
        }

• Click Update trust policy.

In the end you should see something like this: