Can't get g_form.setLabelOf to work

[Daniel103]

I'm trying to use g_form.setLabelOf in an onChange client script on the default Problem form. Here is my very simple code:

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
   if (isLoading || newValue === '') {
      return;
   }

	if(newValue == 'software'){
		g_form.setLabelOf('short_description', 'Problem Statement');
		g_form.setLabelOf('description', 'What is current state?');
		g_form.showFieldMsg('short_description', 'should have changed to PS', 'info');
	}
	else{
		g_form.setLabelOf('short_description', 'still Short description');
		g_form.setLabelOf('description', 'still Description');
		g_form.showFieldMsg('short_description', 'should have changed to SD', 'info');
	}
}

The field is the OOB "category" field. The showFieldMsg calls are just to check that the code is making it to those points, which it is. None of the setLabelOf calls work. When I run setLabelOf in my browser console it also doesn't work and returns false. If I put a nonexistent field into the fieldname variable of setLabelOf then I get an error field-message: 

onChange script error: TypeError: Cannot read property 'id' of undefined function () { [native code] }

Am I doing something wrong? Shouldn't setLabelOf work dynamically without saving or reloading the form?

[SanjivMeher]

Does it work on an onLoad client script?

[Daniel103]

It does work in an onLoad client script.

[ccajohnson]

Are the fields visible on the form. By default, the Short description field is not visible. Also, make sure that you are selecting the correct field to trigger onChange. If you can provide a screen shot of what you have so far it would be helpful.

[Daniel103]

I thought short description was a default field in Problem. Either way, the fields are visible on the form I'm testing on. The client script:

A random problem I'm testing on: