diff --git a/libgphoto2_port/m4/gp-documentation.m4 b/libgphoto2_port/m4/gp-documentation.m4
index b4a486a33..c7af93127 100644
--- a/libgphoto2_port/m4/gp-documentation.m4
+++ b/libgphoto2_port/m4/gp-documentation.m4
@@ -119,9 +119,9 @@ AC_DEFUN([GP_CHECK_DOXYGEN],[dnl
 AC_REQUIRE([GP_CHECK_DOC_DIR])dnl
 AC_PATH_PROG([DOT], [dot], [false])
 AC_PATH_PROG([DOXYGEN], [doxygen], [false])
-AM_CONDITIONAL([HAVE_DOXYGEN], [test "x$DOXYGEN" != "false"])
-AM_CONDITIONAL([HAVE_DOT], [test "x$DOT" != "false"])
-if test "x$DOT" != "false"; then
+AM_CONDITIONAL([HAVE_DOXYGEN], [test "x$DOXYGEN" != "xfalse"])
+AM_CONDITIONAL([HAVE_DOT], [test "x$DOT" != "xfalse"])
+if test "x$DOT" != "xfalse"; then
 	AC_SUBST([HAVE_DOT],[YES])
 else
 	AC_SUBST([HAVE_DOT],[NO])