Call to _api/GroupSiteManager/CreateGroupEx end point (to create Teams Site) causes JSON Node error

[RigidCollision]

Usinbg the following to create a SharePoint Teams Site:

<?php
	error_reporting(E_ERROR);

	// autoload
	spl_autoload_register(function ($classname) {
		$classname = ltrim($classname, "\\");
		preg_match('/^(.+)?([^\\\\]+)$/U', $classname, $match);
		$classname = './'.str_replace("\\", "/", $match[1])
			.str_replace(array("\\", "_"), "/", $match[2])
			.".php";
		include_once $classname;
	});

	use Office365\Runtime\Auth\ClientCredential;
	use Office365\Runtime\Http\RequestOptions;
	use Office365\Runtime\ResourcePath;
	use Office365\SharePoint\ClientContext;
	use Office365\SharePoint\Web;

	// Data variables
	$reference="SIT0000010";
	$site="My Address";

	try {
		// Login credentials
		$credentials = new ClientCredential("MyUserId", "MyUserPass");
		$ctx = (new ClientContext("https://MyDomain.sharepoint.com"))->withCredentials($credentials);

		// Build request
		$url = "{$ctx->getBaseUrl()}/_api/GroupSiteManager/CreateGroupEx";
		$request = new RequestOptions($url);

		//payload
		$payload = array (
			"alias" => "CommunicationSite1",
			"displayName" => "CommunicationSite1",
			"isPublic" => true,
			"optionalParams" => 
			array (
				"CreationOptions" => 
				array ("SPSiteLanguage:1033","implicit_formula_292aa8a00786498a87a5ca52d9f4214a_6142d2a0-63a5-4ba0-aede-d9fefca2c767"),
				"Description" => "MyDescription",
				"Owners" => 
				array (
					"MyUserEmail"
				)
			)
		);

		// Excecute
		$resp = $ctx->getPendingRequest()->executeQueryDirect($request);
		// Capture response
		$json = json_decode($resp->getContent(), true);
		print("Site has been created: {$json["d"]["Create"]["SiteUrl"]} \n");
	}
	catch (Exception $e) {
		echo 'Failed: ',  $e->getMessage(), "\n";
	}
?>

Results in:

Failed: {"error":{"code":"-1, Microsoft.Data.OData.ODataException","message":{"lang":"en-US","value":"A node of type 'StartArray' was read from the JSON reader when trying to read a value of a property; however, a 'PrimitiveValue' or 'StartObject' node was expected."}}}

The JSON checks out as 100% valid.

{
	"alias": "CommunicationSite1",
	"displayName": "CommunicationSite1",
	"isPublic": true,
	"optionalParams": {
		"CreationOptions": ["SPSiteLanguage:1033", "implicit_formula_292aa8a00786498a87a5ca52d9f4214a_6142d2a0-63a5-4ba0-aede-d9fefca2c767"],
		"Description": "MyDescription",
		"Owners": ["MyUserEmail"]
	}
}

A remote collegue has verified that this works in Postman.