diff --git a/src/post_modules/GTK1/configure.in b/src/post_modules/GTK1/configure.in
index c101028d931f654d40e5a17ce682a40f430be26b..21e0d5e8259842310b6232c781be6d55ccebf7c0 100644
--- a/src/post_modules/GTK1/configure.in
+++ b/src/post_modules/GTK1/configure.in
@@ -636,6 +636,8 @@ if test "x$WITH_GTK" = x1 ; then
  fi
  
  AC_CHECK_LIB(gtkdatabox, main,,,`$ac_cv_path_GTK_CONFIG --libs`)
+ # This cache variable interferes with GTK2.
+ unset ac_cv_header_gtkdatabox_h
  AC_CHECK_HEADERS(gtkdatabox.h)
  
  AC_SUBST(pgtk_have_gtkdatabox)