In Unit test I get AutoMapper.AutoMapperMappingException, but code runs

I have a section of code that runs when I fire up my Asp.Net app, but Unit Tests fail.

The code I'm using is here:

var thisResource = db.ProjectSubmission.Find(id);
if (thisResource == null)
{
    return Redirect(Request.UrlReferrer.ToString());
}
// Error during Unit test happens here:
var thisModel = Mapper.Map<ProjectSubmissionViewModel>(thisResource); 

In my AutoMapperConfig.cs file (run during app startup) I map it like this:

Mapper.CreateMap<ProjectSubmission, ProjectSubmissionViewModel>()
    .ForMember(dest => dest.RequestedReviewsCount,
        opt => opt.ResolveUsing<CustomProjectSubmissionResolver>());

public class CustomProjectSubmissionResolver : ValueResolver<ProjectSubmission, int>
{
    protected override int ResolveCore(ProjectSubmission source)
    {
        return source.RequestedReviews.Count;
    }
}

My thisResource.RequestedReviews has one record during the Unit test.

I'm sure I'm missing something, but not sure what.

Here is more of the message I get during the test:

Mapping types:

ProjectSubmission -> ProjectSubmissionViewModel

PublicationSystem.Model.ProjectSubmission -> PublicationSystem.ViewModels.ProjectSubmissionViewModel

Destination path:

ProjectSubmissionViewModel

Source value:

PublicationSystem.Model.ProjectSubmission