diff --git a/doc/CMakeLists.txt b/doc/CMakeLists.txt
index f58d09ec6..0eb36a69f 100644
--- a/doc/CMakeLists.txt
+++ b/doc/CMakeLists.txt
@@ -1,4 +1,4 @@
-add_subdirectory(klipper)
+ecm_optional_add_subdirectory(klipper)
 if(POLKITQT_FOUND)
-  add_subdirectory(PolicyKit-kde)
+  ecm_optional_add_subdirectory(PolicyKit-kde)
 endif()