diff --git a/data/core/common.lua b/data/core/common.lua
index 022dd1dc..5f6e8bc9 100644
--- a/data/core/common.lua
+++ b/data/core/common.lua
@@ -229,7 +229,7 @@ end
 
 
 function common.normalize_path(filename)
-  if PATHSEP == '\\' then
+  if filename and PATHSEP == '\\' then
     filename = filename:gsub('[/\\]', '\\')
     local drive, rem = filename:match('^([a-zA-Z])(:.*)')
     return drive and drive:upper() .. rem or filename