I use the following update:

collection.update(q, new BasicDBObject("$pull", new BasicDBObject("x",null))
//where q is some object which queries the correct doc in the collection and "x" corresponds to an array

and I receive the following error:

2012.01.05 10:34:36 root - Error: java.lang.NullPointerException: can't save a null object
java.lang.NullPointerException: can't save a null object
at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:95)
at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:86)
at com.mongodb.DefaultDBEncoder.writeObject(DefaultDBEncoder.java:27)
at com.mongodb.OutMessage.putObject(OutMessage.java:142)
at com.mongodb.DBApiLayer$MyCollection.update(DBApiLayer.java:345)
at com.mongodb.DBCollection.update(DBCollection.java:165)
at com.mongodb.DBCollection.update(DBCollection.java:197)
at com.mongodb.DBCollection.update(DBCollection.java:209)

The null is created in the first place as a result of using the $unset:

collection.update(q, new BasicDBObject("$unset", new BasicDBObject("x"+".$",1)))
//where q is some object which queries the correct doc in the collection and "x" corresponds to an array

I cannot use the $pull in replacement of $unset because i would require exact matching, which doesn't seem possible as documented:
"Because of this feature, to use the embedded doc as a match criteria, you cannot do exact matches on array elements."

I had assumed that using the null pointer reference was ok based on the post by Andrew Wharton specified in: http://www.mongodb.org/display/DOCS/Java+Types

Is this the correct way to be doing this? The only other way to accomplish this would be to query the entire array, modify it, then insert it back; but I would rather not do this as this could lead to a race condition.