Autowiring a collection via the constructor with Spring

I have what seems to be a simple problem, as stated in the title. Here is the kind of class I have :

public class Foo {
    @Autowired
    public Foo(@Qualifier("bar") Set<String> bar) {
        // ...
    }
}

Which I try to run with the following spring context :

<context:annotation-config />
<util:set id="bar">
    <value>tata</value>
    <value>titi</value>
    <value>toto</value>
</util:set>
<bean id="foo" class="Foo" />

This fails to run with :

No matching bean of type [java.lang.String] found for dependency [collection of java.lang.String]: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Qualifier(value=bar)}

Note that if I add other parameters to my constructor, it works fine. If I use setter injection, it works fine. I'm sure I miss something obvious ... do you know what ?