How to troubleshoot 'Nullable object must have a value' error?

[SoftCircuits]

I have the following query:

var query = DbContext.TripSegments
    .Where(ts => ts.RailcarTrip.WaybillRailcar.Waybill.CompanyCode == companyCode &&
        ts.IsLoaded && ts.EndDate == null &&
        ts.DestinationCity == city && ts.DestinationState == state);

//query = query
//    .GroupBy(ts => ts.RailcarTrip.WaybillRailcar.RailcarNumber)
//    .Select(x => x.First());

int count = await query.CountAsync();

This query works just fine. But if I uncomment the two commented lines, I get an exception.

System.InvalidOperationException: Nullable object must have a value.

RailcarTrip and WaybillRailcar are navigation fields using non-nullable foreign key columns. RailcarNumber is a non-nullable text column.

Beyond that, it's a black box and I don't know how else to narrow down which column or field is causing a problem. How would a person troubleshoot something like this?

More Information

The output window shows only this:

Exception thrown: 'System.InvalidOperationException' in System.Private.CoreLib.dll
Nullable object must have a value.

Here is my stack trace.

at System.ThrowHelper.ThrowInvalidOperationException_InvalidOperation_NoValue()
at System.Nullable`1.get_Value()
at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.ClientProjectionRemappingExpressionVisitor.Visit(Expression expression)
at System.Linq.Expressions.ExpressionVisitor.VisitUnary(UnaryExpression node)
at System.Linq.Expressions.UnaryExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.ClientProjectionRemappingExpressionVisitor.Visit(Expression expression)
at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.ApplyProjection(Expression shaperExpression, ResultCardinality resultCardinality, QuerySplittingBehavior querySplittingBehavior)
at Microsoft.EntityFrameworkCore.Query.Internal.SelectExpressionProjectionApplyingExpressionVisitor.VisitExtension(Expression extensionExpression)
at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryTranslationPostprocessor.Process(Expression query)
at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass12_0`1.<ExecuteAsync>b__0()
at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteAsync[TResult](Expression query, CancellationToken cancellationToken)
at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.ExecuteAsync[TResult](Expression expression, CancellationToken cancellationToken)
at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ExecuteAsync[TSource,TResult](MethodInfo operatorMethodInfo, IQueryable`1 source, Expression expression, CancellationToken cancellationToken)
at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ExecuteAsync[TSource,TResult](MethodInfo operatorMethodInfo, IQueryable`1 source, CancellationToken cancellationToken)
at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.CountAsync[TSource](IQueryable`1 source, CancellationToken cancellationToken)
at TTRailtrax.Controllers.InboundRailcarsController.<BuildInboundRailcars>d__2.MoveNext() in D:\Users\jwood\source\repos\Railtrax\TTRailtrax\Controllers\InboundRailcarsController.cs:line 55

Include provider and version information

EF Core version: 6.0.1
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: .NET 6.0
Operating system: Windows 11
IDE: Visual Studio 2022 17.0.4

[smitpatel]

Duplicate of #26748