"Enumerator failed to MoveNextAsync" when using SingleOrDefaultAsync - after Save

Steps to reproduce

I’m really sorry I don’t have time right now for a full repro, but if this is tricky to track down I can put one together.

When I do a get-or-update:

            var rr = await _context.Set<ResourceReference>()
                .SingleOrDefaultAsync(r => r.Provider == "azure" && r.ResourceId == resourceId, cancel);
            if (rr == null)
            {
                _context.Set<ResourceReference>().Add(rr = new ResourceReference
                {
                    Provider = "azure",
                    ResourceId = resourceId
                });
            }

This works fine.

HOWEVER, if I run the above, THEN SaveChangesAsync, THEN exactly the same code again (same params), then the exception is triggered.

If I don’t have the save call, it works fine.

System.InvalidOperationException: Enumerator failed to MoveNextAsync.
   at Microsoft.EntityFrameworkCore.Query.ShapedQueryCompilingExpressionVisitor.SingleOrDefaultAsync[TSource](IAsyncEnumerable`1 asyncEnumerable, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.ShapedQueryCompilingExpressionVisitor.SingleOrDefaultAsync[TSource](IAsyncEnumerable`1 asyncEnumerable, CancellationToken cancellationToken)

Pretty confused… but even if I create a new context for every call, I still get the error.

Further technical details

EF Core version: 3.0.0 / 3.1.1 ( just checked - still persists) Database provider: Microsoft.EntityFrameworkCore.SqlServer Target framework: NET Core 3.0 Operating system: Windows 10 64-bit IDE: VS Code