diff --git a/cmake/modules/ExternalProject.cmake b/cmake/modules/ExternalProject.cmake
index c09ccaf641e6ceafcbcf05daae6e9091562360e8..29c5e6efae6741212b9753689a7fc7581361b588 100644
--- a/cmake/modules/ExternalProject.cmake
+++ b/cmake/modules/ExternalProject.cmake
@@ -1168,7 +1168,7 @@ function(ExternalProject_Add_Step name step)
     COMMAND ${command}
     COMMAND ${touch}
     DEPENDS ${depends}
-    WORKING_DIRECTORY ${work_dir}
+    $<$<BOOL:${work_dir}>:WORKING_DIRECTORY ${work_dir}>
     VERBATIM
     )