🚀 go-pugleaf

I've run into a strange problem that I can't solve. I want to be able to amend a series of numbers (line numbers), and I place these in a character style so I can easily find them. I thought I could use the following code to do that (all sorts of refinements omitted):

app.findPreferences = null;
app.findPreferences.appliedCharacterStyle = "Counter";
app.changePreferences.changeText = ""
f = app.activeDocument.search( undefined, false, false, undefined );
for( i = 0 ; i < f.length ; i++)
   {
   x = parseInt( f[i].contents );
   x++;
   f[i].contents = x.toString();
   }

This works well as long as all numbers are greater than or smaller than 10. But using this code to increment (or decrement) the range 8-12, for instance, with both single-digit and double-digit numbers, results in odd behaviour, presumably because the debugger shows that single-digit numbers are returned as objects of type Character, but double-digit numbers as objects of type TextStyleRange. Here are some results (first line is the original, second line is the first run, third line, second run, etc.):

Incrementing:

6 7 8 9
7 8 9 10
8 9 10 aN0
9 10 aN0 NN0
10 aN0 NN0 NN0
11 NaN NaN NaN

Decrementing

11 12 13 14
10 11 12 13
9 10 12 12
8 9 11 12
7 8 10 11
6 7 9 10
5 6 8 9

Both incrementing and decrementing give wrong results, but incrementing seems to be worse. Am I doing anything avoidably stupid? And can it be corrected?

Thanks,
Peter