1 files changed, 2 insertions, 2 deletions
diff --git a/acf/model/init.lua b/acf/model/init.lua
index 9d09204..0254fce 100644
--- a/acf/model/init.lua
+++ b/acf/model/init.lua
@@ -112,7 +112,7 @@ function M.Reference:_validate(context, value)
       if not stringy.startswith(value, prefix) then
 	 raise(path, 'Reference out of scope ('..scope..')')
       end
-      value = string.sub(value, string.len(prefix) + 1, -1)
+      value = value:sub(prefix:len() + 1, -1)
    end
 
    -- assume one-level ref for now
@@ -158,7 +158,7 @@ end
 
 function M.Collection:auto_ui_name(name)
    if not name then return end
-   if string.sub(name, -1, -1) ~= 's' then name = name..'s' end
+   if name:sub(-1, -1) ~= 's' then name = name..'s' end
    return super(self, M.Collection):auto_ui_name(name)
 end