Problems with EqulatyComparer

I have a custom generic comparer like this:

internal class SyncItemComparer : IEqualityComparer<ISyncItem>
{
    public bool Equals(ISyncItem x, ISyncItem y)
    {
        return x.GlobalID.Equals(y.GlobalID);
    }

    public int GetHashCode(ISyncItem obj)
    {
        return obj.GlobalID.GetHashCode();
    }
}

Then i try to use it in this method:

void Sync<ISyncItem>(IEnumerable<ISyncItem> masterItems,
IEnumerable<ISyncItem> clientItems)
{
    var comparer = new SyncItemComparer();

    foreach (var master in masterItems)
    {
        if(clientItems.Contains<ISyncItem>(master, comparer))
        {
            
        }
    }
}

The compiler warns on the Contains method:
Argument type 'MyApp.Business.Comparers.SyncItemComparer' is not assignable
to 'System.Collections.Generic.IEqualityComparer<ISyncItem>`

But the SyncItemComparer implements
System.Collections.Generic.IEqualityComparer<ISyncItem>!

It is very late and I must be blind to something ......