PipelineDefinitionBuilder.Match() method generates incorrect BsonDocument for equality filter

I have the following code:

PipelineDefinitionBuilder.Match<BsonDocument, BsonDocument>(myPipeline, Builders<BsonDocument>.Filter.Eq("$_id", "$$userId"));

I expect the generated BsonDocument to be:

{{

{ "$eq": ["$_id", "$$userId"] }

}}

Instead I get this:

{{

{ "$_id": "$$userId" }

}}

Which results in the aggregation command to fail with error:

Command aggregate failed: unknown top level operator: $_id

Other filters seems to work as expected. e.g

PipelineDefinitionBuilder.Match<BsonDocument, BsonDocument>(myPipeline, Builders<BsonDocument>.Filter.Gt("$var1", "$$var2"));

Generates:

{{

{ "$gt": ["$var1", "$$var2"] }

}}