title: What does this Entity Framework Core exception mean?
body: <p>To setup the exception message here is some code.</p>
  
  <p>In my Repository class I have the following code:</p>
  
  <pre><code>public async Task&lt;CoverLetterBank&gt; GetCoverLetterBankAsync(int clBankID)
  {
      IQueryable&lt;CoverLetterBank&gt; query = BankContext.CoverLetterBanks.Include(c =&gt; c.CoverLetters).Include(d =&gt; d.CoverLetterCategories);
  
      return await query.FirstOrDefaultAsync(c =&gt; c.Id == clBankID);
  }
  
  public async Task&lt;IEnumerable&lt;CoverLetterBank&gt;&gt; GetCoverLetterBanksAsync()
  {
      return await BankContext.CoverLetterBanks.ToListAsync(); ;
  }
  </code></pre>
  
  <p>The GetCoverLettersBanksAsync() call works.  However, when I call the GetCoverLetterBankAsync() method I get the following exception:</p>
  
  <p>"   at System.ThrowHelper.ThrowNotSupportedException(ExceptionResource resource)\r\n   at System.Collections.ObjectModel.ReadOnlyCollection<code>1.System.Collections.Generic.ICollection&lt;T&gt;.Add(T value)\r\n   at Microsoft.EntityFrameworkCore.Metadata.Internal.ClrICollectionAccessor</code>3.Add(Object entity, Object value, Boolean forMaterialization)\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.AddToCollection(INavigation navigation, InternalEntityEntry value, Boolean forMaterialization)\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalMixedEntityEntry.AddToCollection(INavigation navigation, InternalEntityEntry value, Boolean forMaterialization)\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.AddToCollection(InternalEntityEntry entry, INavigation navigation, InternalEntityEntry value, Boolean fromQuery)\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.SetReferenceOrAddToCollection(InternalEntityEntry entry, INavigation navigation, InternalEntityEntry value, Boolean fromQuery)\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.ToDependentFixup(InternalEntityEntry dependentEntry, InternalEntityEntry principalEntry, IForeignKey foreignKey, Boolean fromQuery)\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.InitialFixup(InternalEntityEntry entry, Boolean fromQuery)\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.TrackedFromQuery(InternalEntityEntry entry)\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.TrackedFromQuery(InternalEntityEntry entry)\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.MarkUnchangedFromQuery()\r\n   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.StartTrackingFromQuery(IEntityType baseEntityType, Object entity, ValueBuffer&amp; valueBuffer)\r\n   at Microsoft.EntityFrameworkCore.Query.QueryContext.StartTracking(IEntityType entityType, Object entity, ValueBuffer valueBuffer)\r\n   at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.CustomShaperCompilingExpressionVisitor.g__ProcessCurrentElementRow|9_0TIncludingEntity,TIncludedEntity\r\n   at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.CustomShaperCompilingExpressionVisitor.PopulateIncludeCollection[TIncludingEntity,TIncludedEntity](Int32 collectionId, QueryContext queryContext, DbDataReader dbDataReader, ResultCoordinator resultCoordinator, Func3 parentIdentifier, Func<code>3 outerIdentifier, Func</code>3 selfIdentifier, Func<code>5 innerShaper, INavigation inverseNavigation, Action</code>2 fixup, Boolean trackingQuery)\r\n   at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.CustomShaperCompilingExpressionVisitor.g__ProcessCurrentElementRow|9_0TIncludingEntity,TIncludedEntity\r\n   at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.CustomShaperCompilingExpressionVisitor.PopulateIncludeCollection[TIncludingEntity,TIncludedEntity](Int32 collectionId, QueryContext queryContext, DbDataReader dbDataReader, ResultCoordinator resultCoordinator, Func3 parentIdentifier, Func<code>3 outerIdentifier, Func</code>3 selfIdentifier, Func<code>5 innerShaper, INavigation inverseNavigation, Action</code>2 fixup, Boolean trackingQuery)\r\n   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable<code>1.Enumerator.MoveNext()\r\n   at System.Collections.Generic.LargeArrayBuilder</code>1.AddRange(IEnumerable<code>1 items)\r\n   at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable</code>1 source)\r\n   at System.Linq.Enumerable.ToArray[TSource](IEnumerable<code>1 source)\r\n   at System.Linq.SystemCore_EnumerableDebugView</code>1.get_Items()"</p>
  
  <p>Here are the frameworks and packages:</p>
  
  <p><a href="https://i.stack.imgur.com/CynRz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CynRz.png" alt="enter image description here"></a></p>