diff -urNad a/mapunion.c a/mapunion.c.orig
--- a/mapunion.c	2011-05-31 08:51:45.691271866 +0200
+++ a/mapunion.c.orig	2011-05-31 08:51:35.083376517 +0200
@@ -240,7 +240,7 @@
 }
 
 /* clean up expression tokens */
-void msUnionLayerFreeExpressionTokens(layerObj *layer)
+int msUnionLayerFreeExpressionTokens(layerObj *layer)
 {
     int i,j;
     freeExpressionTokens(&(layer->filter));