Java 8 call to generic method is ambiguous

I'm porting Java7 code to Java8, and I came up with a following problem. In my codebase I have two methods:

public static <T> ImmutableSet<T> append(Set<T> set, T elem) {
    return ImmutableSet.<T>builder().addAll(set).add(elem).build();
}

public static <T> ImmutableSet<T> append(Set<T> set, Set<T> elemSet) {
    ImmutableSet.Builder<T> newSet = ImmutableSet.builder();
    return newSet.addAll(set).addAll(elemSet).build();

Compiler returns error about ambiguous match for method append in following test:

@Test(expected = NullPointerException.class)
public void shouldAppendThrowNullPointerForNullSecondSet() {
    ImmutableSet<Integer> obj = null;
    CollectionUtils.append(ImmutableSet.of(1), obj);
}

Compiler error:

reference to append is ambiguous both method append(java.util.Set,T) in CollectionUtils and method append(java.util.Set,java.util.Set) in CollectionUtils match

How to rewrite these functions to work with type inference from introduced with Java8?