diff --git a/build_files/cmake/macros.cmake b/build_files/cmake/macros.cmake
index 0aad121ea98..c942d2d86ac 100644
--- a/build_files/cmake/macros.cmake
+++ b/build_files/cmake/macros.cmake
@@ -1460,12 +1460,15 @@ macro(find_python_module_file
     )
     if(${out_var_abs})
       set(_${out_var_abs}_DEPS "${_python_mod_file_deps_test}" CACHE STRING "")
-      string(LENGTH "${_python_root}" _python_root_len)
-      string(SUBSTRING ${${out_var_abs}} ${_python_root_len} -1 ${out_var_rel})
-      unset(_python_root_len)
     endif()
   endif()
 
+  if(${out_var_abs})
+    string(LENGTH "${_python_root}" _python_root_len)
+    string(SUBSTRING ${${out_var_abs}} ${_python_root_len} -1 ${out_var_rel})
+    unset(_python_root_len)
+  endif()
+
   unset(_python_mod_file_deps_test)
   unset(_python_base)
   unset(_python_root)