Ambiguous Invocation IQueryable or IEnumerable

This problem is normally caused by a failure in type inference from in the expression provided to the Where statement. As was mentioned in the comments above it is 100% caused by the assignment operator in the lambda returning an int instead of a bool.

To be clear – where you have

var result = db.Set.Where(x => x.Id = num).Select(whatever);

You should have

var result = db.Set.Where(x => x.Id == num).Select(whatever);

Another good (and more common) example of this is something like this

public class Elem 
{
    public bool IsSomething {get;set;}                    
    public bool? IsSomethingElse {get;set;}
}

Then if you do the following query, which looks very reasonable at fist glance, it will fail to compile with the rather puzzling error of “abiguous invocation”

IQueryable<Elem> queryable = GetQueryable();
var result = queryable.Where(e => e.IsSomething && e.IsSomethingElse).ToList();

If you weren’t writing this statement inside a lambda then you would get a more meaningful error of

“Cannot apply operator ‘&&’ to operands of type ‘System.Nullable<bool>’ and ‘bool'”

Which would immediately tell you that you are failing to return a boolean.