diff --git a/pisi/build.py b/pisi/build.py
index b50007d6..158ce116 100644
--- a/pisi/build.py
+++ b/pisi/build.py
@@ -424,7 +424,7 @@ class Builder:
                 import catbox
                 # stupid autoconf family needs /usr/lib/conftest* and /usr/lib/cf* for some conftest,
                 # http://sources.gentoo.org/viewcvs.py/portage/trunk/sandbox/files/sandbox/sandbox.c also permits these
-                valid_dirs = [self.pkg_dir(), "/tmp/", "/var/tmp/", "/dev/tty", "/dev/pts/", "/dev/pty", "/dev/null", "/dev/zero", "/proc/", "/usr/lib/conftest", "/usr/lib/cf"]
+                valid_dirs = [self.pkg_dir(), "/tmp/", "/var/tmp/", "/dev/tty", "/dev/pts/", "/dev/pty", "/dev/null", "/dev/zero", "/dev/ptmx", "/proc/", "/usr/lib/conftest", "/usr/lib/cf"]
                 if ctx.config.values.build.buildhelper == "ccache":
                     valid_dirs.append("%s/.ccache" % os.environ["HOME"])
                 # every qt/KDE application check these