Volley﹕ [37470] NetworkDispatcher.run: Unhandled exception java.lang.NullPointerException

<pre name="code" class="java">{first_name=null, fb_id=XXXX, email=null, name=Yuan, link=null, last_name=null, gender=null, device_id=""}

E/Volley﹕ [37470] NetworkDispatcher.run<span style="color:#ffffcc;">:<span style="background-color: rgb(204, 102, 0);"> Unhandled exception java.lang.NullPointerException
java.lang.NullPointerException</span></span>
at libcore.net.UriCodec.encode(UriCodec.java:132)
at java.net.URLEncoder.encode(URLEncoder.java:57)
at android.volley.Request.encodeParameters(Request.java:406)
at android.volley.Request.getBody(Request.java:392)
at android.volley.toolbox.HurlStack.addBodyIfExists(HurlStack.java:223)
at android.volley.toolbox.HurlStack.setConnectionParametersForRequest(HurlStack.java:210)
at android.volley.toolbox.HurlStack.performRequest(HurlStack.java:106)
at android.volley.toolbox.BasicNetwork.performRequest(BasicNetwork.java:82)
at android.volley.NetworkDispatcher.run(NetworkDispatcher.java:105)

protected Map<String, String> getParams(){
Map<String, String> params = new HashMap<String, String>();
params.put("key1", value1);
params.put("key2",value2);
params.put("key3",value3);
return <span style="color:#ff0000;">checkParams</span>(params);
}; <span style="color:#ff0000;">

</span>   private Map<String, String> <span style="color:#ff0000;">checkParams</span>(Map<String, String> map){
Iterator<Entry<String, String>> it = map.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<String, String> pairs = (Map.Entry<String, String>)it.next();
if(pairs.getValue()==null){
map.put(pairs.getKey(), "");
}
}
return map;
} <span style="color:#ff0000;">
</span>