g.getFont does not return font set by g.setFont

can anybody explain the following behaviour (entered into the left-hand side of the Web IDE):

>g.setFont12x20();
=Graphics: {
  flip: function () { [native code] },
  drawRoundedRect: function (x1,y1,x2,y2,r) { ... },
  fillRoundedRect: function (x1,y1,x2,y2,r) { ... }
 }
>g.getFont();
="Custom"

I have a similar problem deep inside a program which crashes with

Uncaught Error: Font bitmap must be a String
 at line 1 col 1140
...t!=null){g.setFont(Details.font);}g.setFontAlign(xAlignment,yAli...

In Details.font I've stored a previously configured font, obtained using g.getFont() (which returned Custom, although the font was set using g.setFont12x20() before)