Setting project name "debug_msgs" occurs error while "mydebug_msgs" is ok

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when I build my msg project whose name is "debug_msgs" with ros2 bouncy, I get error as follow:

command:

   colcon build

output:

Starting >>> debug_msgs
--- stderr: debug_msgs                         
CMake Error at /root/ros2_ws/install/ament_cmake_export_libraries/share/ament_cmake_export_libraries/cmake/ament_export_libraries.cmake:56 (message):
  ament_export_libraries() package 'debug_msgs' passes the build
  configuration keyword 'debug_msgs__rosidl_generator_c' as the last exported
  library
Call Stack (most recent call first):
  /root/ros2_ws/install/rosidl_generator_c/share/rosidl_generator_c/cmake/rosidl_generator_c_generate_interfaces.cmake:171 (ament_export_libraries)
  /root/ros2_ws/install/ament_cmake_core/share/ament_cmake_core/cmake/core/ament_execute_extensions.cmake:38 (include)
  /root/ros2_ws/install/rosidl_cmake/share/rosidl_cmake/cmake/rosidl_generate_interfaces.cmake:169 (ament_execute_extensions)
  CMakeLists.txt:23 (rosidl_generate_interfaces)


---
Failed   <<< debug_msgs [ Exited with code 1 ]

Summary: 0 packages finished [1.21s]

but after I changed the project name to "mydebug_msgs", It goes well, could any one tell me why ? the test demo is : https://github.com/lxbeyond/bouncy_msg