diff --git a/cddl/usr.sbin/zfsd/Makefile.common b/cddl/usr.sbin/zfsd/Makefile.common
index 2f1b7b9f5a32..60c889651aad 100644
--- a/cddl/usr.sbin/zfsd/Makefile.common
+++ b/cddl/usr.sbin/zfsd/Makefile.common
@@ -30,7 +30,7 @@ INCFLAGS+= -I${SRCTOP}/sys/cddl/contrib/opensolaris/uts/common
 INCFLAGS+= -I${SRCTOP}/sys/cddl/contrib/opensolaris/uts/common/fs/zfs
 INCFLAGS+= -I${SRCTOP}/cddl/usr.sbin
 
-CFLAGS= -g -DNEED_SOLARIS_BOOLEAN ${INCFLAGS}
+CFLAGS+= -g -DNEED_SOLARIS_BOOLEAN ${INCFLAGS}
 
 LIBADD+=	devdctl zfs zfs_core util geom bsdxml sbuf nvpair uutil