diff --git a/definitions/grib2/templates/template.4.optical.def b/definitions/grib2/templates/template.4.optical.def
index 8d9b9002f..2c35a583c 100644
--- a/definitions/grib2/templates/template.4.optical.def
+++ b/definitions/grib2/templates/template.4.optical.def
@@ -18,7 +18,7 @@ constant billion = 1000000000 : hidden;
 meta firstWavelengthInNanometres  multdouble(firstWavelength, billion) : read_only;
 meta secondWavelengthInNanometres multdouble(secondWavelength, billion) : read_only;
 
-if (defined(MTG2Switch) and MTG2Switch == 0) {
+if (defined(MTG2Switch) and MTG2Switch != 0) {
  # typeOfWavelengthInterval (table 4.91 specifies if first / second / first+second limit(s) are to be defined)
  if ( typeOfWavelengthInterval == 0 || typeOfWavelengthInterval == 3 || typeOfWavelengthInterval == 5 || typeOfWavelengthInterval == 8 || typeOfWavelengthInterval == 11 ){
   alias mars.wavelength = firstWavelengthInNanometres ;