cannot configure "gaussian blur" video effect

Changing parameter value for "spatial blur" video effect ("spatialisation" in french) does not work.

With vlc running from terminal, I get the following error log message(s) :

[0x17a4438] qt4 interface error: Module gaussianblur's gaussianblu-sigma variable is of an unsupported type ( 0 )
[0x17a4438] qt4 interface error: Module gaussianblur's gaussianblu-sigma variable is of an unsupported type ( 0 )
[0x17a4438] qt4 interface error: Module gaussianblur's gaussianblu-sigma variable is of an unsupported type ( 0 )
[0x17a4438] qt4 interface error: Module gaussianblur's gaussianblu-sigma variable is of an unsupported type ( 0 )
...