diff --git a/package/flex/Config.in b/package/flex/Config.in
index f74a0f0e8b..1ecd0c3572 100644
--- a/package/flex/Config.in
+++ b/package/flex/Config.in
@@ -12,5 +12,7 @@ config BR2_PACKAGE_FLEX_BINARY
 	# needs fork()
 	depends on BR2_USE_MMU
 	depends on BR2_PACKAGE_FLEX
+	# runtime dependency
+	select BR2_PACKAGE_M4
 	help
 	  Install the flex binary tool in the target filesystem.
diff --git a/package/flex/flex.mk b/package/flex/flex.mk
index f7e319f1c4..748365b020 100644
--- a/package/flex/flex.mk
+++ b/package/flex/flex.mk
@@ -11,6 +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
 # we don't have a host-gettext/libintl
 HOST_FLEX_DEPENDENCIES = host-m4