Check the table. You probably have others.
$Name: 162 Empty, 0, 0.50, 0
$Name: 163 Empty, 0, 0.50, 0
$Name: 164 Empty, 0, 0.50, 0
$Name: 165 Empty, 0, 0.50, 0
$Name: 166 Empty, 0, 0.50, 0
$Name: 167 Empty, 0, 0.50, 0
$Name: 168 Empty, 0, 0.50, 0
$Name: 169 Empty, 0, 0.50, 0
$Name: 170 Empty, 0, 0.50, 0
$Name: 171 Empty, 0, 0.50, 0
$Name: 172 Empty, 0, 0.50, 0
Any of those might work. They might crash it too . You can always add an actual blank .wav file. That is definitely going to work.
The bigger problem is that FRED is allowing you to enter -1 in the first place. It shouldn't. explosion-effect says that the argument must be positive but FRED2 isn't checking that for reasons to do with compatibility, etc.