diff --git a/src/lua-env-safe.lua b/src/lua-env-safe.lua
index 2249c07..079bd8a 100644
--- a/src/lua-env-safe.lua
+++ b/src/lua-env-safe.lua
@@ -62,6 +62,9 @@ local disallowed_exts = tmap {
 -- Return: clean file path if allowed (or nil if disallowed),
 --         error string (or nil if allowed)
 local function safe_path(fn, readonly)
+  if type(fn) ~= 'string' then
+    return nil, 'Filename must be a string'
+  end
   fn = fn:gsub('\\', '/')
   fn = fn:gsub('^ +', '')
   fn = fn:gsub(' +$', '')