title: How to solve entity framework core System.ArgumentNullException
body: <p>From the following stacktrace, I can't tell what is wrong by the error message. I ran the migration command with <code>--verbose</code> flag hoping it would help pinpoint the issue but zilch.</p>
  
  <pre><code>  dotnet ef migrations add migwl3 --verbose
  </code></pre>
  
  <p>I can't find a parameter named <code>key</code></p>
  
  <pre><code>  System.ArgumentNullException: Value cannot be null.
    Parameter name: key
     at Microsoft.EntityFrameworkCore.Utilities.Check.NotNull[T](T   value, String parameterName)
     at     Microsoft.EntityFrameworkCore.SqlServerMetadataExtensions.SqlServer(IKey key)
     at    Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.    &lt;Add&gt;d__41.MoveNext()
     at System.Linq.Enumerable.&lt;CastIterator&gt;d__35`1.MoveNext()
     at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source)
     at     Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.   &lt;Add&gt;d__29.MoveNext()
     at     Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.   &lt;DiffCollection&gt;d__56`1.MoveNext()
     at System.Linq.Enumerable.ConcatIterator`1.MoveNext()
     at      Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.S      ort(IEnumerable`1 operations, DiffContext diffContext)
    at      Microsoft.EntityFrameworkCore.Migrations.Design.MigrationsScaffolder.Scaf      foldMigration(String migrationName, String rootNamespace, String      subNamespace)
     at     Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMig      ration(String name, String outputDir, String contextType)
    at      Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(S    tring name, String outputDir, String contextType)
    at     Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.&lt;&gt;c__DisplayClass3_0`1.&lt;Execute&gt;b__0()
    at     Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Exec     ute(Action action)
   Value cannot be null.
   Parameter name: key
  </code></pre>
  
  <p>Any tips on how to zero in on the issue (the project compiles fine) would be greatly appreciated</p>