Challenge herself

[Dragonlette]

I am currently writing reports for school and I want to give a target of
"Challenge herself/himself by attempting extension tasks whenever possible"

Word seems to think there is an issue with using himself/herself in this sentence, so please can you knowledgeable people help me out with how to phrase that a bit better?
Thanks

I'd say that's fine. Word can be a bit of an idiot.

Thanks. I couldn't see anything wrong with it, but I'm aware that I've written about a bazillion reports now and even very normal words and phrases are causing my brain to frazzle. So I'm glad to have a second opinion and I'm a bit to ask my colleagues.

Word is awful. Its a US based program to begin with. Half the time its sentence structure suggestions sound like yoda after a heavy Jack Daniel's session.