diff --git a/CMakeLists.txt b/CMakeLists.txt
index a30bcc3b..63e5585c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -111,7 +111,7 @@ if(VIENNALS_USE_VTK)
   enable_language(C)
 
   find_package(VTK QUIET)
-  set(VIENNALS_SYSTEM_VTK ${VTK_FOUND})
+  set(VIENNALS_SYSTEM_VTK ${VTK_FOUND} CACHE INTERNAL "")
 
   CPMFindPackage(
     NAME VTK
@@ -147,8 +147,6 @@ if(VIENNALS_USE_VTK)
             # This fixes an issue with the install script that causes find_package to fail.
             "CMAKE_PROJECT_NAME VTK")
 
-  target_compile_definitions(${PROJECT_NAME} INTERFACE VIENNALS_USE_VTK=1)
-
   target_link_libraries(
     ${PROJECT_NAME}
     INTERFACE VTK::CommonExecutionModel