Expanding list inside CMake generator expression

Question

I have the following snippet of a CMakeLists.txt file:

set(MAIN_SOURCES
  src/main.cpp
  src/main.hpp
)

set(FEATURE_SOURCES
  src/feature.cpp
  src/feature.hpp
)

option(ENABLE_FEATURE "Enable feature in app" ON)

add_executable(app)

target_sources(app PRIVATE
  ${MAIN_SOURCES}
  $<$<BOOL:${ENABLE_FEATURE}>:${FEATURE_SOURCES}>
)

When trying to configure, I an error like that:

CMake Error in CMakeLists.txt:
  Cannot find source file:

    src/feature.hpp>

Then I heard that when there are spaces inside the generator expression, the whole generator needs to be quoted. However according to CMake manual, FEATURE_SOURCES is a list (space-separated list?). I would assume the variable doesn't contain spaces when expanded.

Nonetheless I also tried quoting the whole thing:

target_sources(app PRIVATE
  ${MAIN_SOURCES}
  "$<$<BOOL:${ENABLE_FEATURE}>:${FEATURE_SOURCES}>"
)

which yields again the same error.

Any suggestions how can I get this generator expression right?