context.select in ProxyProvider0.update not allowed?

[taosimple]

ProxyProvider0<TransformedCounter>(
          update: (context, value) {
            final value = context.select<Counter, int>((counter) => counter.value);
            return TransformedCounter(value * 2);
          },
        ),

---

ProxyProvider0<TransformedCounter>(
          update: (context, value) {
            final counter = context.watch<Counter>();
            return TransformedCounter(counter.value * 2);
          },
        ),

Above codes use context.select will throw a exception: Tried to use context.select outside of the build method of a widget., but use context.watch is fine, why context.select is not allowed, it more efficient than context.watch.