[Groff] why this warning ?

[walter harms]

hi list,
i have a warning that i do not understand.
the tbl below give a warning: grotty:tab:7: character above first line discarded
(using: groff -t -Tascii tab). Any idea why ?

.TS
tab(|);
l s s s
c c c c.
UEBERSCHRIFT
a|b|c|d
a|b|c|d
a|b|c|d
.TE

i use groff 1.18.1 from suse 10.0

re,
 walter