Unable to create a constant value of type 'System.Object' in Entity Framework

I have a table named UserTenders having many-to-one relationship with aspnet_Membership table.
I am using EntityFramework 4.0 and when I try something like this, it errors.

var tenders = ctx.UserTenders
    .Where(tender => tender.HasAdminApproved.Equals(true))
    .ToList();

The error is

System.NotSupportedException
Unable to create a constant value of type 'System.Object'.
Only primitive types ('such as Int32, String, and Guid') are supported in this context.

This snippet below works.

var tenders = ctx.UserTenders.ToList();

What could be wrong in my code? Feel like I am missing something very trivial.

I would like to filter all those rows that have the bit field HasAdminApproved as true