PowerMock @PrepareForTest throws exception for a class containing a sorted call [JAVA 8]

Question

I have a class(TempClass) containing a map that uses a static method of another class(UtilClass) to generate a list using this map. For mocking the static method(can't avoid using the static method), I'm using Powermockito. However, I'm getting this exception when I try to run the test.

javassist version used: 3.18.1-GA

java.lang.IllegalStateException: Failed to transform class with name somepackage.UtilClass. Reason: javassist.bytecode.InterfaceMethodrefInfo cannot be cast to javassist.bytecode.MethodrefInfo

it disappears when I remove a sorted call from the static method

I know it can be resolved by dependency changes mentioned here. But I don't want to change the dependency. My question is more around why is this happening?

This is a simplified example of a code that I'm using.

TempClass.java

import com.google.common.collect.Lists;
import com.google.common.collect.Maps;

import java.util.List;
import java.util.Map;

public class TempClass {

    public void execute(){
        List<Integer> integerList1 = Lists.newArrayList();
        List<Integer> integerList2 = Lists.newArrayList();
        integerList1.add(10);
        integerList1.add(1);
        integerList1.add(4);

        integerList2.add(10);
        integerList2.add(20);
        integerList2.add(30);

        Map<String, List<Integer>> stringListMap = Maps.newHashMap();
        stringListMap.put("AA", integerList1);
        stringListMap.put("AB", integerList2);

        List<Integer> sortedList = UtilClass.getSomeList(stringListMap);
    }

}

UtilClass.java

import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class UtilClass {

  public static List<Integer> getSomeList(Map<String, List<Integer>> inputs) {
    return inputs.entrySet().stream()
        .sorted(Map.Entry.comparingByKey())
        .map(Map.Entry::getValue)
        .flatMap(Collection::stream)
        .collect(Collectors.toList());
  }
}

TempClassTest.java

import org.junit.Assert;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest({
    UtilClass.class
})
public class TempClassTest {

    @Test
    public void dummy(){
        Assert.assertEquals(1, 1);
    }
}

The exception disappears and the test runs fine when I remove the sorted call from the method.

import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class UtilClass {

  public static List<Integer> getSomeList(Map<String, List<Integer>> inputs) {
    return inputs.entrySet().stream()
        .map(Map.Entry::getValue)
        .flatMap(Collection::stream)
        .collect(Collectors.toList());
  }
}