[R] Igraph: family 'serif' not included in PostScript device

[Knut Krueger]

Does anybody know how to solve this error?

postscript(file= "file.ps", family = "Helvetica", font = "Helvetica")
plot.igraph(g, layout=layout.circle, vertex.label.font=2)
dev.off()
error in  text.default(x, y, labels = labels, col = label.color, family 
= label.family,  :
  family 'serif' not included in PostScript device


Regards Knut