diff --git a/configure.ac b/configure.ac
index 8dcc9c0a9b..e14ec99e5e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -407,7 +407,7 @@ AC_SUBST(CLIENTLIBS)
 CLIENTGUIFLAGS="-DNOCLIPBOARD -DNOTASKBAR"
 
 if test "${enable_debug}" = yes; then
-    CLIENTGUIFLAGS="$CLIENTGUIFLAGS -D__WXDEBUG__"
+    CLIENTGUIFLAGS="${CLIENTGUIFLAGS} -D__WXDEBUG__"
 fi
 
 CLIENTGUILIBS=${WX_LIBS}