Incorrect "Incompatible return type Object in lambda expression"

Using Intellij 13.1.4 and Java 8

Given the source code:

package my.foobar; import java.util.concurrent.ConcurrentMap;import java.util.stream.Collectors; import com.google.common.collect.ImmutableList;import com.google.common.collect.ImmutableSet;import lombok.ToString; publicclass FooBar {     @ToString    publicstaticclass Foo {        private String foo;        private String bar;         public Foo(String foo, String bar) {            this.foo = foo;            this.bar = bar;        }         public String getFoo() {            return foo;        }         publicvoid setFoo(String foo) {            this.foo = foo;        }         public String getBar() {            return bar;        }         publicvoid setBar(String bar) {            this.bar = bar;        }    }     publicstaticvoid main(String[] args) {        final ImmutableList<Foo> foos = ImmutableList.of(new Foo("a", "b"), new Foo("b", "c"));        final ConcurrentMap<Foo, ImmutableSet<String>> map = foos.stream()                .collect(Collectors.toConcurrentMap(                        it -> it, // Incorrect "Incompatible return type Object in lambda expression" error                        it -> ImmutableSet.of(it.getBar()),                        (a, b) -> ImmutableSet.<String>builder().addAll(a).addAll(b).build()                ));        System.out.println("map = " + map); // the error appears not immediatelly, but after writing some more code, after the lambda expression is written    }} 

which compiles, runs and produces output:

map = {FooBar.Foo(foo=b, bar=c)=[c], FooBar.Foo(foo=a, bar=b)=[b]}

But Intellij shows an errror at

it -> it