diff --git a/cmake/QtIRHelpers.cmake b/cmake/QtIRHelpers.cmake
index 8e2af7f8..678848f6 100644
--- a/cmake/QtIRHelpers.cmake
+++ b/cmake/QtIRHelpers.cmake
@@ -117,7 +117,7 @@ function(qt_ir_get_args_from_optfile_configure_filtered optfile_path out_var)
     if(submodules)
         qt_ir_get_top_level_submodules(include_submodules exclude_submodules)
         if(NOT include_submodules AND arg_ALREADY_INITIALIZED)
-            set(include_submodules "${submodules}")
+            string(REPLACE "," ";" include_submodules "${submodules}")
         endif()
 
         # qtrepotools is always implicitly cloned, but it doesn't actually