Unable to set WRKSRC

I'm trying to update math/abs, the lastest
distfile is named abs-src-0908.tar.gz, on extraction
gives dir abs-src-0.908 (0908->0.908 ) . I guess the WRKSRC
by default is set to ${WRKDIR}/${DISTNAME}, as you can see
this gives an error when applying patches, resetting
WRKSRC = ${WRKDIR}/abs-src-0.908 doesn't work, the error
persists. Can anyone explain why WRKSRC can't be set.

Makefile
# $OpenBSD: Makefile,v 1.20 2019/07/12 21:02:23 sthen Exp $

COMMENT= free spreadsheet with graphical user interface

DISTNAME= abs-src-0908
PKGNAME= ${DISTNAME:S/-src//}
REVISION= 0
CATEGORIES= math

PERMIT_PACKAGE= Yes
WANTLIB= X11 Xaw Xext Xmu Xpm Xt c>=35 m

MASTER_SITES= http://utenti.gufi.org/~gmarco/files/distfiles/ \
http://fresh.t-systems-sfr.com/unix/src/privat2/

WRKSRC= ${WRKDIR}/abs-src-0.908
NO_TEST= Yes

do-install:
${INSTALL_DATA_DIR} ${PREFIX}/share/examples/abs
${INSTALL_PROGRAM} ${WRKSRC}/abs ${PREFIX}/bin
${INSTALL_DATA} ${WRKSRC}/Examples/* ${PREFIX}/share/examples/abs

.include <bsd.port.mk>

One can easily get rid of the error, by renaming the distfile
and hosting it on openbsd ftp server, since the earlier
version of math/abs is available on it. I've already tried
renaming distfile setting DISTNAME=abs-src-0.908, and the
update builds successfully.