Default Help generating incremental OTA update package

Hi guys,

I am trying to generate an incremental OTA update package from AOSP source code.

I saw that the key may be the file ./build/tools/releasetools/ota_from_target_files that requires the "-i <previous_OTA_zip_file>" parameter to do this.

I tried adding this in the ./build/core/Makefile correct section, changing :

Code:
$(INTERNAL_OTA_PACKAGE_TARGET): $(BUILT_TARGET_FILES_PACKAGE) $(DISTTOOLS)
	@echo "Package OTA: $@"
	$(hide) ./build/tools/releasetools/ota_from_target_files -v \
           -p $(HOST_OUT) \
	   -k $(KEY_CERT_PAIR) \
	   $(BUILT_TARGET_FILES_PACKAGE) $@
to

Code:
$(INTERNAL_OTA_PACKAGE_TARGET): $(BUILT_TARGET_FILES_PACKAGE) $(DISTTOOLS)
	@echo "Package OTA: $@"
	$(hide) ./build/tools/releasetools/ota_from_target_files -v \
	   -i ./temp/old_ota.zip \
           -p $(HOST_OUT) \
	   -k $(KEY_CERT_PAIR) \
	   $(BUILT_TARGET_FILES_PACKAGE) $@
However, when I issue the command 'make otapackage', I receive the following error :
... can't find recovery API version in input target-files

Does anyone know how to fix this ? Or, if I'm doing something wrong, please enlighten me

Thanks/