diff --git a/m4/ac_python_devel.m4 b/m4/ac_python_devel.m4
index eee6374..7e7e6e4 100644
--- a/m4/ac_python_devel.m4
+++ b/m4/ac_python_devel.m4
@@ -65,7 +65,7 @@ to something else than an empty string.
                	print distutils.sysconfig.get_python_inc();"`
 		AC_MSG_RESULT([$python_path])
 	if test -z "${python_path}"; then
-		AC_MSG_WARN([cannot get include path. Please set it by hand.])
+		AC_MSG_WARN([none. Please set PYTHON_CPPFLAGS by hand.])
 	else
 		python_path="-I$python_path"
 	fi