Entity Framework Count not raising exception

I'm using LINQ to return a count of items from Entity Framework. I've noticed, however, that despite an invalid DSN being set, I'm not getting an exception raised, simply a 0 return value.

So, I've overridden my database context like this to pass in a DSN:

public partial class MissingDetails : DbContext
{
    public MissingDetails(string DSN) : base(DSN)
        //: base("name=MissingDetails")
    {
    }

I then call it:

        public int ImportCount
        {
            get
            {
                dbContext = new MissingDetails(suppliedDSN);
                return dbContext.Details.Count(
                    i => (
                        (i.UniqueGroupReference == uniqueIdentifier) &&
                        (i.UniqueGroupReferenceSubGroup == subIdentifier)
                        )
                    );
            }
        }

All works fine when I supply a valid DSN, but if I give it something invalid it just returns 0 rather than an exception. Is this as expected?

btw, I'm targetting .Net 4.0 so using Entity Framework 6.2.0

thanks.