I figured it out. I was using a bad example I found in the past of how to map query string to the method parameters.
In case anyone else needs it, in order to have optional parameters in a query string such as:
you would use:
[Route("products/filter/{apc?}/{xpc?}/{sku?}")]
public IHttpActionResult Get(string apc = null, string xpc = null, int? sku = null)
{ ... }
It seems odd to have to define default values for the method parameters when these types already have a default.