Say ‘-PT’ or ‘-PL’ to inform FWEAVE that its output will be processed by TeX or LaTeX, respectively. Beginning with Version 1.50, the default processor is LaTeX (‘-PL’). If you always use TeX, it’s easiest to put ‘-PT’ into the .fweb initialization file.
Please note that ‘-PT’ is no longer supported; FWEB development is now based exclusively on LaTeX.