There is a white background problem when SVG is converted to PNG.

[BlackBerryXSQ]

my development environment is :
windows 7
gcc (Rev2, Built by MSYS2 project) 9.2.0
GraphicsMagick 1.3.34
go 1.13.8

when i use mw.SetImageBackgroundColor() function , There is a white background problem when SVG is converted to PNG.
on svg, the background is transparent , i convert svg to png , on png,the background is white . I hope the generated PNG is as transparent as SVG.
not matter how i try to set the background opacity or color,it doesn't work.
Is this a bug or am I using it the wrong way?
thank you.