lib with subdirectories build failed

[19504643]

I'm building a project, there are more than one libraries, so I move your example lib files to its subdirectory like following

lib
├── Kconfig
├── log_print
│   ├── Kconfig
│   ├── list_debug.c
│   ├── log_print.c
│   └── Makefile
└── Makefile

cat lib/Makefile

lib-$(CONFIG_ENABLE_LOGGING) += log_print/

cat lib/Kconfig

source "lib/log_print/Kconfig"

cat lib/log_print/Makefile

lib-$(CONFIG_ENABLE_LOGGING) += log_print.o
lib-$(CONFIG_DEBUG_LIST) += list_debug.o

lib/log_print/Kconfig stays the same as original lib/Kconfig
lib/log_print/Makefile stays the same as original lib/Makefile

but error occurs when making, need your help, thanks

AR lib/lib.a
ar: lib/log_print/: file format not recognized
make[1]: *** [scripts/Makefile.build:237: lib/lib.a] Error 1
make: *** [Makefile:369: lib] Error 2