rosbridge /rosjs/typeStringFromTopic does not work

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I call this service (/rosjs/typeStringFromTopic) with the following code:

var types=[];
con.callService('/rosjs/typeStringFromTopic', '[/mav2/fcu/mag]', function(rsp) {
alert("here");
alert(rsp);
types.push(rsp); });

My topic is not empty but I "here" is never alerted. There is the following error in my rosbridge terminal

Problem Problem Problem Problem Problem Problem Problem Problem Problem Problem
Traceback (most recent call last):
File "/data/my_ros/rosbridge/bin/rosbridge.py", line 39, in handleFrameHelper call = json.loads(frame)
File "/usr/lib/python2.7/json/__init__.py", line 326, in loads return _default_decoder.decode(s)
File "/usr/lib/python2.7/json/decoder.py", line 366, in decode obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
Problem Problem Problem Problem Problem Problem Problem Problem Problem Problem

Thank you for your help :)

Benoît