HS/FS Python invalid literal for float()

[747driver]

I made a module which converts the temperatures sent by a Mobotix camera (strings) into a Float.
Some examples of the format of the temperature sent by Mobotix:
+11.4
+ 9.2 (with a space between the + en the 9)
- 8.3 (with a space between the - en the 8)
-11.5

The logic formula looks like this:
5012|0|"EN[1][:1]=='+'"|"float(EN[1][-4:])" |""|1|0|0|0
5012|0|"EN[1][:1]<>'+'"|"0-float(EN[1][-4:])"|""|1|0|0|0

For some reason this seems to work but creates Exceptions in the HS3 debug file:
File "./../hssrc/hs_logik.py", line 232, in calc
File "formula", line 1, in [module]
ValueError: invalid literal for float(): ----
[5012L, 10032L, 0L, "EN[1][:1][]'+'", '0-float(EN[1][-4:])', '', 1L, 0L, 0L, 0L]

Any idea why?