Why am I getting this error: "Argument 1 passed to myFunction() must be an instance of SimpleXMLElement, null given"?

Question

I keep getting this error:

Argument 1 passed to myFunction() must be an instance of SimpleXMLElement, null given

Here is my code:

function myFunction(SimpleXMLElement $order_status_response_element) {
    // Get any invoices firstly.
    $invoice_details_element_array = $order_status_response_element->xpath('//Detail/RefInfo/RefIDQual[text()="IN"]/parent::*/parent::*');
    $invoice_numbers_element_array = $order_status_response_element->xpath('//Detail/RefInfo/RefIDQual[text()="IN"]/parent::RefInfo/RefID');
    $non_invoice_details_element_array = $order_status_response_element->xpath('//Detail[not(RefInfo/RefIDQual[text()="IN"])]');
    if(count($non_invoice_details_element_array) > 1) throw new Exception('More than one instance of non-invoiced details in array! Aborting!');
    $non_invoice_details_element = $non_invoice_details_element_array[0];
    echo("non_invoice_details_element is a SimpleXMLElement, true? ".is_a($non_invoice_details_element,'SimpleXMLElement')."\n");


    $non_invoice_detail = sXMLtoArray($non_invoice_details_element);
    ^^^ERROR HAPPENS HERE^^^
}

/**
* Return an array, given a SimpleXMLElement object.
*
* @param SimpleXMLElement $elem
* @return array
*/
function sXMLtoArray(SimpleXMLElement $elem) {
    return json_decode(json_encode($elem),TRUE);
} 

Now the output I get looks like this:

non_invoice_details_element is a SimpleXMLElement, true? 1

Argument 1 passed to sXMLtoArray() must be an instance of SimpleXMLElement, null given, called in {path} on line 2033 and defined {paths}

So if is_a() tells me that that indeed it's a SimpleXMLElement then why does it think it's null when it gets passed to the sXMLtoArray() function? If it wasn't a SimpleXMLElement then it would throw an exception anyway previously because the $non_invoice_details_element_array would be zero-length since the xpath wouldn't exist.

What the heck is wrong here?