diff --git a/thirdparty/detray/CMakeLists.txt b/thirdparty/detray/CMakeLists.txt
index 1188da9fca1..4f58b537601 100644
--- a/thirdparty/detray/CMakeLists.txt
+++ b/thirdparty/detray/CMakeLists.txt
@@ -15,7 +15,7 @@ message( STATUS "Building Detray as part of the Acts project" )
 set( DETRAY_VERSION "v${_acts_detray_version}")
 
 # Declare where to get Detray from.
-FetchContent_Declare( Detray ${ACTS_DETRAY_SOURCE} )
+FetchContent_Declare( -DDetray ${ACTS_DETRAY_SOURCE} )
 
 add_definitions( DETRAY_ALGEBRA_ARRAY )