db.test.insert(

{"_id": "someid", "attribute": "somevalue"}

);
db.test.update(

{"_id" : "someid"}

,{$set: {"pos_list":

{"PRP$": "PRP$ - Possessive pronoun"}

}});

The second line returns the following error:
not okForStorage

Interestingly, inserting the same attribute-value pair works:
db.test.insert(

{"_id": "someotherid", "PRP$": "PRP$ - Possessive pronoun"}

);