zoo plotting - invalid 'ylim' value

Hi all,

I am very new to 'zoo', so this could probably be a trivia question,
but I got the above error when I plot my zoo object

plot(stb.foreign) #this work fine

stb.foreign$room.con.lai

2009-05-28  2009-05-29  2009-06-01  2009-06-02  2009-06-03
2009-06-04  2009-06-05
11104740    10772400    10775110    10397160    10037930     9986480
 10003780
 2009-06-08  2009-06-09  2009-06-10  2009-06-11  2009-06-12
2009-06-15  2009-06-16
10163340    10545920    11020540    10720430    10484040    10502080
 10932930
 2009-06-17
11201700

plot(stb.foreign$room.con.lai)

Error in plot.window(...) : invalid 'ylim' value

Thanks

-k