[CMake] INSTALL( EXPORT ... requires target that is not in export set

[Hugh Sorby]

I am trying to use INSTALL( EXPORT ... but failing.  I have a project 
like this

ImageMagick/CMakeLists.txt -- 1.
ImageMagick/magick/CMakeLists.txt -- 2.
ImageMagick/coders/CMakeLists.txt -- 3.

1. calls ADD_SUBDIRECTORY for both 2 and 3
2. has a target "magick" that is a library
3. has a target "coders" that is a library

2 needs 3 so in 2 I have the line

TARGET_LINK_LIBRARIES( magick coders )

then I have

INSTALL( TARGETS magick EXPORT magick-targets ARCHIVE
     DESTINATION lib )

and then I try

INSTALL( EXPORT magick-targets DESTINATION lib/cmake )

which doesn't fly and I get the error message

CMake Error: INSTALL(EXPORT "MagickCore-targets" ...) includes target 
"MagickCore" which requires target "coders" that is not in the export set.

Is there someone who can enlighten me on how this works I have read the 
documentation but I am unclear (as in it's not crystal to me).