calling javascript function from C

I would like to call a javascript function from c and use the return value.
In first test:

/*JSON{
  "type" : "function",
  "name" : "ExecJS",
  "generate" : "jswrap_ExecJS",
  "params" : [
    ["function","JsVar","A Function or String to be executed"]
  ],
  "return" : ["JsVar","can be used for everything"]
}
*/
JsVar *jswrap_ExecJS(JsVar *func){
  JsVar x = *jspExecuteFunction(func, 0,0,0);
  jsWarn("na sowas: %d\n",x);
  jsvUnLock(&x);
  jsWarn("done\n");
}

got this error, after ExecJS(function(){return 99;}); whats wrong ?

WARNING: na sowas: 99
WARNING: done
=ASSERT(v->flags == JSV_UNUSED) FAILED AT src/jsvar.c:399
#1[r1,l2] Object {

#2[r1,l2] Name String [1 blocks] "\xFF"      #3[r1,l2] Object { 
    #6[r1,l2] Name String [2 blocks] "timers"          #8[r2,l1] Array(0) [ ]
    #9[r1,l2] Name String [2 blocks] "watches"          #11[r2,l1] Array(0) [ ]
    #13[r1,l2] Name String [1 blocks] "net"          #14[r1,l2] String [1 blocks] "\x05\x00\x00\x00\x00\xFF\xFF\xFF"
    #15[r1,l2] Name String [2 blocks] "history"          #25[r1,l1] Array(2) [
        #26[r1,l2] Name Integer 0              #19[r1,l1] String [5 blocks] "console.log(\"<\",\"<<\",JSON.stringify(process.env),\">>\",\">\")"
        #224[r1,l2] Name Integer 1              #213[r1,l1] String [3 blocks] "ExecJS(function(){return 99;});"
      ]
  } 
#17[r1,l2] Name String [2 blocks] "console"      #18[r1,l1] NativeFunction 0x40123264 (1) { }
#28[r1,l2] Name String [1 blocks] "JSON"      #27[r1,l1] NativeFunction 0x401232ac (1) { }
#33[r1,l2] Name String [2 blocks] "process"      #32[r1,l1] NativeFunction 0x40123210 (1) { }

}
HALTING.