HUGO IN函数不接受变量作为ITEM

I am using the IN function of Hugo that can be used as in SET ITEM as defined at Hugo Documentation for IN

I am trying to filter out posts of a particular category by the below code

{{ range where .Site.Pages "Section" "products"}}
   {{ if in .Params.categories "New Arrival" }}
          <li>{{ .Title }}</li>
   {{end}}
{{end}}

the above code works completely fine.

But if I use a variable instead of a string for the ITEM. It always returns False.

{{ $display_product_cat := "New Arrival" }}

{{ range where .Site.Pages "Section" "products"}}
      {{ if in .Params.categories $display_product_cat }}
          <li>{{ .Title }}</li>
      {{end}}
{{end}}

The above code does not work as expected.

Wondering if I am missing something here. Appreciate your time and help.