c# contravariance and covariance: cannot convert from implementation to interface

Question

Experts, what is wrong here:

I have these interfaces:

public interface IEvent {}

public interface ISingleEventModelChangeInterrogator<in TEvent> where TEvent : IEvent
{
    List<string> GetChangedCandidates(object context, TEvent changedEventArgs);
    bool IsInterestedIn(TEvent changedEventType);
}

Then, I have this base class:

public abstract class SingleEventModelChangeInterrogator<TEvent> : ISingleEventModelChangeInterrogator<TEvent>
        where TEvent : IEvent
    {
        public abstract List<string> GetChangedCandidates(object context, TEvent changedEventArgs);

        public bool IsInterestedIn(TEvent changedEventType)
        {
            return changedEventType.GetType() == typeof(TEvent);
        }
    }

And the following implementations:

    public class PriceChangeEvent : IEvent { }

    public class PurchaseOrderAffectedByPriceChangeInterrogator : SingleEventModelChangeInterrogator<PriceChangeEvent >
    {
        public override List<string> GetChangedCandidates(object context, PriceChangeEvent changedEventArgs)
        {
            throw new System.NotImplementedException();
        }
    }

When I attempt to add an instance of PurchaseOrderAffectedByPriceChangeInterrogator to a list of ISingleEventModelChangeInterrogator<IEvent>, as shown below:

 public class RetailEventCoordinator
    {
        public void Initialize()
        {
            var interrogators = new List<ISingleEventModelChangeInterrogator<IEvent>>();
            interrogators.Add(new PurchaseOrderAffectedByPriceChangeInterrogator());
        }
    }

I get the following error:

cannot convert 'model.materialization.service.interrogators.PurchaseOrderAffectedByPriceChangeInterrogator' to 'model.materialization.service.interrogators.ISingleEventModelChangeInterrogator<dotnet.model.materialization.service.events.IEvent>

What am I missing here?