CMake BOOL generator expression evaluates an empty list to true

Question

I'm trying to add a custom command target to my build, and I'm trying to append a list of arguments based on a variable which contains itself a generator expression. My code looks like this:

  list(
    APPEND MY_INCLUDE_DIRECTORIES
      $<TARGET_PROPERTY:target,INCLUDE_DIRECTORIES>
      $<TARGET_PROPERTY:target,INTERFACE_INCLUDE_DIRECTORIES>
  )

  add_custom_target(
    custom_target
    COMMAND binary
      "$<$<BOOL:$<GENEX_EVAL:${MY_INCLUDE_DIRECTORIES}>>:--includes;$<JOIN:${MY_INCLUDE_DIRECTORIES},;--includes;>>"
    COMMAND_EXPAND_LISTS
    VERBATIM
  )

However, when MY_INCLUDE_DIRECTORIES evaluates to an empty list, it tries to execute binary --includes and I don't understand why. I've modified part of the custom target to debug the output of each generator expression as such.

  add_custom_target(
    custom_target
    COMMAND binary 
      "$<BOOL:$<GENEX_EVAL:${MY_INCLUDE_DIRECTORIES}>>"
      "$<GENEX_EVAL:${MY_INCLUDE_DIRECTORIES}>"
    COMMAND_EXPAND_LISTS
    VERBATIM
  )

But it prints the command binary 1, which implies that the boolean expression is true, but the list is empty. Am I doing something wrong?

EDIT: I've found a fix, although I still don't understand what was happening. I had to append the generator expressions as such:

  list(
    APPEND MY_INCLUDE_DIRECTORIES
      "$<TARGET_PROPERTY:target,INCLUDE_DIRECTORIES>$<TARGET_PROPERTY:target,INTERFACE_INCLUDE_DIRECTORIES>"
  )