Error when trying to convert nested for loop to Java 8 streams

I am trying to convert a nested for loop to java 8 streams.

Loop implementation

private Set<String> getAllowRolesFromInheritedPolicy(String userId, List<Policy> allowedPoliciesThisCustomer) {

    Set<String> allowedRolesThisUser = Sets.newHashSet();

    for (Policy policy : allowedPoliciesThisCustomer) {
        Map<String, Role> roles = policy.getRoles();
        for (Role role : roles.values()) {
            if (role.getUsers().contains(userId)) {
                allowedRolesThisUser.add(role.getRoleName());                   
            }
        }
    }

In the code role.getUsers() returns List<String>.

Stream Implementation

I am trying to change the for loop into java 8 streams :

Set<String> allowedRolesThisUser = Sets.newHashSet(allowedPoliciesThisCustomer.stream()
                                       .map(policy -> policy.getRoles().values())
                                       .filter(role -> role.getUsers().contains(userId))
                                       .collect(Collectors.toList()));

But compiler says:

error: cannot find symbol : .filter(role -> role.getUsers().contains(userId))
symbol:   method getUsers(), location: variable role of type Collection<Role>

Role has the function getUsers, Collection<Role> does not. What should I do to make this conversion correct?

Thank you.