diff --git a/cmake/modules/user_cache.cmake b/cmake/modules/user_cache.cmake
index 1e8b59d675..ab6b2dfdac 100644
--- a/cmake/modules/user_cache.cmake
+++ b/cmake/modules/user_cache.cmake
@@ -59,7 +59,7 @@ function(find_appropriate_cache_directory dir)
     if(DEFINED ENV{${env_var}})
       set(env_dir $ENV{${env_var}})
 
-      set(test_user_dir ${env_dir}/${env_suffix_${env_var}})
+      string(JOIN "/" test_user_dir ${env_dir} ${env_suffix_${env_var}})
 
       execute_process(COMMAND ${PYTHON_EXECUTABLE}
         ${ZEPHYR_BASE}/scripts/build/dir_is_writeable.py ${test_user_dir}