Can't create budget action with misconfigured execution role - php

Question

I'm trying to create a budget action via API, and I first create the execution role, and I create it like this:
$createdRole = $iamClient->createRole([
'RoleName' => 'budgets-rds-execution-role',
'AssumeRolePolicyDocument' => json_encode([
'Version' => '2012-10-17',
'Statement' => [
[
'Effect' => 'Allow',
'Action' => 'sts:AssumeRole',
'Principal' => [
'Service' => 'budgets.amazonaws.com',
],
],
[
'Effect' => 'Allow',
'Action' => 'sts:AssumeRole',
'Principal' => [
'Service' => $resource . '.amazonaws.com', // resource is either ec2 or rds
],
],
]
]),
'tags' => [
[
'Key' => 'third-party',
'Value' => 'cloud-kill-switch',
]
],
]);
And it creates successfully, but then, when I try and create the subsequent budget action, I get the following error:
"Error executing "CreateBudgetAction" on "https://budgets.amazonaws.com"; AWS HTTP error: Client error: `POST https://budgets.amazonaws.com` resulted in a `400 Bad Request` response:
{"__type":"AccessDeniedException","Message":"Budgets permission required to assume [ExecutionRole: arn:aws:iam::33519752 (truncated...)
AccessDeniedException (client): Budgets permission required to assume [ExecutionRole: arn:aws:iam::335197525879:role/budgets-rds-execution-role]. Please follow the instruction to grant assumeRole access to [Service Principal: budgets.amazonaws.com]. - {"__type":"AccessDeniedException","Message":"Budgets permission required to assume [ExecutionRole: arn:aws:iam::335197525879:role/budgets-rds-execution-role]. Please follow the instruction to grant assumeRole access to [Service Principal: budgets.amazonaws.com]."}"
All of the questions I've seen say that you need to add the budgets.amazonaws.com principal, but it's already there. Any thoughts?