Syntax error in mfile, but not incommand window

I have a custom class RFile that overloads subsref. When I write in the command window RFile('some_file').save('x','y','-v7.3'), it works well without a warning. But when I put that instruction in a .m file and execute it, I got the error

??? Static method or constructor invocations cannot be indexed.
Do not follow the call to the static method or constructor with
any additional indexing or dot references.

Why the command window didnt raise an error in the first time ? Are there any workaround ?