Implicit conversions weirdness

I am trying to understand why exactly an implicit conversion is working in one case, but not in the other. Here is an example:

   case class Wrapper[T](wrapped: T)
   trait Wrapping { implicit def wrapIt[T](x: Option[T]) = x.map(Wrapper(_))

   class NotWorking extends Wrapping { def foo: Option[Wrapper[String]] = Some("foo") }

   class Working extends Wrapping { 
      def foo: Option[Wrapper[String]] = {
        val why = Some("foo")
        why
      }
    }

Basically, I have an implicit conversion from Option[T] to Option[Wrapper[T]], and am trying to define a function, that returns an optional string, that gets implicitly wrapped.

The question is why, when I try to return Option[String] directly (NotWorking above), I get an error (found : String("foo") required: Wrapper[String]), that goes away if I assign the result to a val before returning it.

What gives?