Magento 2 REST API: Custom API with customer_token expecting string[] instead of string

Question

I made an API which returns wishlist of a user using customer_token as authentication, obtained via this API:

rest/V1/integration/customer/token

My webapi.xml looks something like:

        <?xml version="1.0"?>
    <routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Webapi:etc/webapi.xsd">
     <route url="/V1/fetch_wishlist/" method="GET">
            <service class="Vendor\WishlistApi\Api\WishlistInterface" method="setData"/>
            <resources>
                <resource ref="self" />
            </resources>
            <data>
                <parameter name="customerId" force="true">%customer_id%</parameter>
            </data>
        </route>
    </routes>

But I am getting an error as:

{
"message": "Invalid type for value: \"string\". Expected Type: \"string[]\".",
"trace": null
}

Rest of my code is correct, I used this API with anonymous resource in webapi.xml and it works fine if i send the payload as:

{"customerId": [3500]}

However I get the same exact error if I put the payload as:

{"customerId": 3500}

Any suggestions?

Answer 1

Fixed it by including these comments over the function in the interface:

/**
 * GET for wishlist api
 * @param string $customerId
 * @return string
 */