catkin_make_isolated: which has been found in [path] but it is not a catkin package

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I am building from source with an isolated catkin workspace. I wonder why it throws "not a catkin package", although catkin_make_isolated is supposed to build plain cmake projects, too.

-- Using PYTHON_EXECUTABLE: /usr/bin/python
-- Python version: 2.7
-- Using Debian Python package layout
-- Using CATKIN_ENABLE_TESTING: ON
-- Call enable_testing()
-- Using CATKIN_TEST_RESULTS_DIR: /ws/catkin/build_isolated/eigen_conversions/test_results
-- Found gtest sources under '/usr/src/gtest': gtests will be built
-- catkin 0.5.88
CMake Error at /ws/catkin/src/catkin/cmake/catkin_package.cmake:176 (message):
  catkin_package() CATKIN_DEPENDS on 'orocos_kdl', which has been found in
  '/ws/catkin/build_isolated/orocos_kdl/devel/orocos_kdl-config.cmake', but
  it is not a catkin package
Call Stack (most recent call first):


  /ws/catkin/src/catkin/cmake/catkin_package.cmake:98 (_catkin_package)
  CMakeLists.txt:10 (catkin_package)


-- Configuring incomplete, errors occurred!
See also "/ws/catkin/build_isolated/eigen_conversions/CMakeFiles/CMakeOutput.log".
See also "/ws/catkin/build_isolated/eigen_conversions/CMakeFiles/CMakeError.log".
make: *** [cmake_check_build_system] Error 1
<== Failed to process package 'eigen_conversions': 
  Command '/ws/catkin/devel_isolated/geometry_msgs/env.sh make cmake_check_build_system' returned non-zero exit status 2