diff --git a/package/flex/flex.mk b/package/flex/flex.mk
index 748365b020..236153674e 100644
--- a/package/flex/flex.mk
+++ b/package/flex/flex.mk
@@ -11,7 +11,7 @@ FLEX_LICENSE = FLEX
 FLEX_LICENSE_FILES = COPYING
 FLEX_DEPENDENCIES = \
 	$(if $(BR2_PACKAGE_GETTEXT_IF_LOCALE),gettext) host-m4
-FLEX_CONF_ENV = M4=/usr/bin/m4
+FLEX_CONF_ENV = ac_cv_path_M4=/usr/bin/m4
 # we don't have a host-gettext/libintl
 HOST_FLEX_DEPENDENCIES = host-m4