Error in search... options: expected array, got string

I am trying to use search to find any file with txt extension inside a particular path. I get the error: "Error in call to API function "files/search_v2": options: expected array, got string".  What am I doing wrong here:

 

 

 

    $options = array(
        "filename_only" => true,
		"max_results" => 1,
		"file_status" => "active",
		"path" => "/some/folder/folder",
		"file_extensions" => "txt"
    );

    $params = array(
        "options" => $options,
        "query" => "txt",
    );

    $headers = array(
        'Authorization: Bearer ' . $auth_token,
        'Content-Type: application/json'
    );

    $ch = curl_init();
    curl_setopt( $ch, CURLOPT_POSTFIELDS, json_encode( $params ) );
    curl_setopt( $ch, CURLOPT_RETURNTRANSFER, true );
    curl_setopt( $ch, CURLOPT_CUSTOMREQUEST, "POST" );
    curl_setopt( $ch, CURLOPT_HTTPHEADER, $headers );
    curl_setopt( $ch, CURLOPT_URL, 'https://api.dropboxapi.com/2/files/search_v2' );