Neo4j http API NullPointerException -

i using http api query neo4j server. exact same queries different values not work consistently. infact entire system breaks because of nullpointer exception being thrown. cannot figure out root of problem

{        "query":"start n=node( { current_user_node } ), n1 = node( { contact_node } ) create unique n-[:has_contact {device: {device_id}, name: {name} }]->(n1)",       "params":{"current_user_node":2,"contact_node":5941,"device_id":"f1485935-48f8-4624-af5d-67529ae91227","name":"samir coll "} } 

the above query returns

{     "exception": "nullpointerexception",     "fullname": "java.lang.nullpointerexception",     "stacktrace": [] } 

i tried above query in neo4j-shell command line , query returned null.

while

{     "query":"start n=node( { current_user_node } ), n1 = node( { contact_node } ) create unique n-[:has_contact {device: {device_id}, name: {name} }]->(n1)",     "params":{"current_user_node":1,"contact_node":5658,"device_id":"fa2c589a-6ab5-4d78-aded-7146ca71d0fc","name":"jayesh new"} } 

the above returns

{ "columns": [], "data": [] } 

the data empty relationship exists.

i running neo4j 2.0.0 stable. nodes mentioned in above queries valid. unsure how proceed this. appreciate if can problem.