diff --git a/src/openvpn/tun_afunix.c b/src/openvpn/tun_afunix.c
index 4d48a31..e6f2be1 100644
--- a/src/openvpn/tun_afunix.c
+++ b/src/openvpn/tun_afunix.c
@@ -53,6 +53,8 @@
     const char *msgprefix = "ERROR: failure executing process for tun:";
     struct argv argv = argv_new();
 
+    /* we should always called with a proper unix: dev node string */
+    ASSERT(dev_node && strncmp(dev_node, "unix:", strlen("unix:")) == 0);
     /* since we know that dev-node starts with unix: we can just skip that
      * to get the program name */
     const char *program = dev_node + strlen("unix:");