Error for plotting unitted data.frame when logging one axis

[jordansread]

plot(gpp.disch$disch, gpp.disch$gpp, log='x')
Error in unitted_Ops(.Generic, e1, unitted(e2, NA)) : 
  Units mismatch in comparison 'e1 (ft^3 s^-1) <= e2 ()'

units of disch and gpp are different, yes. But no error for plot(gpp.disch$disch, gpp.disch$gpp, log='')