Currently we have number of places where SerializerFinder adopts approach similar to:

if (IsNotKnown(node) && IsKnown(leftExpression, out var leftSerializer))
{
    IBsonSerializer itemSerializer = leftSerializer.GetItemSerializer();
    AddNodeSerializer(node, itemSerializer);
}

The problem is in case if leftSerializer is IUnknowableSerializer, then IsKnown will return true and GetItemSerializer will throw an error, instead of deducing another IUnknowableSerializer.