NullReferenceException when inserting with Dapper

When I run the following code I get a NullReferenceException saying that an object reference not set to an instance of the object. I've successfully inserted with dapper using less complex objects but the same format, so am not sure what I'm doing wrong.

public void Foo(IEnumerable<FogbugzCase> cases) 
{
    // using a singleton for the SqlConnection
    using (SqlConnection conn = CreateConnection())
    {
        foreach (FogbugzCase fogbugzCase in cases)
        {
            conn.Execute("INSERT INTO fogbugz.Cases(CaseId, Title, ProjectId, CategoryId, Root, MilestoneId, Priority, Status, EstimatedHours, ElapsedHours, AssignedTo, ResolvedBy, IsResolved, IsOpen, Opened, Resolved, Uri, ResolveUri, OutlineUri, SpecUri, ParentId, Backlog) VALUES(@BugId, @Title, @ProjectId, @CategoryId, @RootId, @MilestoneId, @Priority, @StatusId, @EstimatedHours, @ElapsedHours, @PersonAssignedToId, @PersonResolvedById, @IsResolved, @IsOpen, @Opened, @Resolved, @Uri, @ResolveUri, @OutlineUri, @Spec, @ParentId, @Backlog);", new {BugId = fogbugzCase.BugId, Title = fogbugzCase.Title, ProjectId = fogbugzCase.Project.Id, CategoryId = fogbugzCase.Category.Id, RootId = fogbugzCase.Root, MilestoneId = fogbugzCase.Milestone.Id, Priority = fogbugzCase.Priority, StatusId = fogbugzCase.Status.Id, EstimatedHours = fogbugzCase.EstimatedHours, ElapsedHours = fogbugzCase.ElapsedHours, PersonAssignedToId = fogbugzCase.PersonAssignedTo.Id, PersonResolvedById = fogbugzCase.PersonResolvedBy.Id, IsResolved = fogbugzCase.IsResolved, IsOpen = fogbugzCase.IsOpen, Opened = fogbugzCase.Opened, Resolved = fogbugzCase.Resolved, Uri = fogbugzCase.Uri, OutlineUri = fogbugzCase.OutlineUri, Spec = fogbugzCase.Spec, ParentId = fogbugzCase.ParentId, Backlog = fogbugzCase.Backlog});
        }
    }
}

I first tried doing the simpler way of just passing in fogbugzCase instead of the anonymous object but resulted in a different exception about CategoryId.

Anyone see what I'm missing?

It sounds like one of the properties is null. Split the code:

var args =  new {BugId = fogbugzCase.BugId, Title = fogbugzCase.Title, ProjectId = fogbugzCase.Project.Id, CategoryId = fogbugzCase.Category.Id, RootId = fogbugzCase.Root, MilestoneId = fogbugzCase.Milestone.Id, Priority = fogbugzCase.Priority, StatusId = fogbugzCase.Status.Id, EstimatedHours = fogbugzCase.EstimatedHours, ElapsedHours = fogbugzCase.ElapsedHours, PersonAssignedToId = fogbugzCase.PersonAssignedTo.Id, PersonResolvedById = fogbugzCase.PersonResolvedBy.Id, IsResolved = fogbugzCase.IsResolved, IsOpen = fogbugzCase.IsOpen, Opened = fogbugzCase.Opened, Resolved = fogbugzCase.Resolved, Uri = fogbugzCase.Uri, OutlineUri = fogbugzCase.OutlineUri, Spec = fogbugzCase.Spec, ParentId = fogbugzCase.ParentId, Backlog = fogbugzCase.Backlog});
conn.Execute("INSERT INTO fogbugz.Cases(CaseId, Title, ProjectId, CategoryId, Root, MilestoneId, Priority, Status, EstimatedHours, ElapsedHours, AssignedTo, ResolvedBy, IsResolved, IsOpen, Opened, Resolved, Uri, ResolveUri, OutlineUri, SpecUri, ParentId, Backlog) VALUES(@BugId, @Title, @ProjectId, @CategoryId, @RootId, @MilestoneId, @Priority, @StatusId, @EstimatedHours, @ElapsedHours, @PersonAssignedToId, @PersonResolvedById, @IsResolved, @IsOpen, @Opened, @Resolved, @Uri, @ResolveUri, @OutlineUri, @Spec, @ParentId, @Backlog);", args);

It the code fails on the first line, it isn't anything to do with dapper - it didn't even get anywhere near dapper. You'll need to check all the nested members.