diff --git a/simgear/nasal/misc.c b/simgear/nasal/misc.c
index 73bd19d2..acdca6a3 100644
--- a/simgear/nasal/misc.c
+++ b/simgear/nasal/misc.c
@@ -135,6 +135,7 @@ naRef naNil()
 naRef naNum(double num)
 {
     naRef r;
+    r.ref.reftag = ~NASAL_REFTAG;
     r.num = num;
     return r;
 }