The test below fails with
assert: [null] != [null] are equal : undefined
Error: Printing Stack Trace
at printStackTrace (src/mongo/shell/utils.js:37:7)
at doassert (src/mongo/shell/assert.js:6:1)
at Function.assert.neq (src/mongo/shell/assert.js:44:1)
at c:\Users\sridhar\Desktop\ShardKeyUnset.js:39:8

Expect the document to be unchanged
{ "_id" : -1, "ckey" :

, "val" : -1 }
Instead the update goes through resulting in
{ "_id" : -1, "ckey" : { }, "val" : -100 }