diff --git a/processor/main.m4 b/processor/main.m4
index 185f27f..4cf7bb8 100644
--- a/processor/main.m4
+++ b/processor/main.m4
@@ -188,7 +188,7 @@ define(`LIST_DIFF', `dnl
 foreach(`i', `($1)', `pushdef(`item_'defn(`i'))')dnl allocate variable for items in $1 
 foreach(`i', `($2)', `pushdef(`item_'defn(`i'))undefine(`item_'defn(`i'))')dnl allocate variable for items in $2, and undefine it 
 foreach(`i', `($1)', `ifdef(`item_'defn(`i'), `defn(`i') ')')dnl see what is still defined
-foreach(`i', `($2)', `popdef(`item_'defn(`i'))')dnl
+foreach(`i', `($2)', `define(`item_'defn(`i'))popdef(`item_'defn(`i'))')dnl
 foreach(`i', `($1)', `popdef(`item_'defn(`i'))')dnl
 ')
 dnl }}}