\s[] gives syntax error (should restore previous size)

[Jim Avera]

#!/bin/sh
# \s[] causes a syntax error.  According to 'info groff'
# \s with no argument should restore the previous point size
# Such behavior would be consistent with \m \f and \F, so this seems 
likely to be a bug

# (using groff 1.22.2 build from cvs)

groff -Tps <<'EOF' >/dev/null
.warn
.ps 10
.tm Before: .s=\n[.s]
original size \s[20] larger \s[] previous size
.tm After: .s=\n[.s]
.if !\n[.s]=10 .ab bug .s=\n[.s] -- \\s[] did not restore previous point 
size