Potentially dangerous request is loading the page

Question

I hosted an application in ASP.NET 4.0 webforms.

When I am browsing hostname/DressDetail.aspx?<script>alert(313)</script>&category=1&code=76 It is showing an error message like: A potentially dangerous Request.QueryString value was detected from the client (="alert(313)

But I am expecting same error message as above when I am browsing hostname/DressDetail.aspx?<script>alert(313)</script>=1&category=1&code=76

But it loads the page. But it should not be. The only difference is an extra =1 in the latter url.