diff --git a/src/main/java/com/google/devtools/common/options/Converters.java b/src/main/java/com/google/devtools/common/options/Converters.java
index 56bc63c2a4b272..15e8b479db0b29 100644
--- a/src/main/java/com/google/devtools/common/options/Converters.java
+++ b/src/main/java/com/google/devtools/common/options/Converters.java
@@ -250,8 +250,8 @@ static String joinEnglishList(Iterable<?> choices) {
     return buf.length() == 0 ? "nothing" : buf.toString();
   }
 
-  public static class SeparatedOptionListConverter implements Converter<List<String>> {
-
+  /** Converter for a list of options, separated by some separator character. */
+  public static class SeparatedOptionListConverter implements Converter<ImmutableList<String>> {
     private final String separatorDescription;
     private final Splitter splitter;
     private final boolean allowEmptyValues;
@@ -264,8 +264,8 @@ protected SeparatedOptionListConverter(
     }
 
     @Override
-    public List<String> convert(String input) throws OptionsParsingException {
-      List<String> result =
+    public ImmutableList<String> convert(String input) throws OptionsParsingException {
+      ImmutableList<String> result =
           input.isEmpty() ? ImmutableList.of() : ImmutableList.copyOf(splitter.split(input));
       if (!allowEmptyValues && result.contains("")) {
         // If the list contains exactly the empty string, it means an empty value was passed and we
diff --git a/src/test/java/com/google/devtools/common/options/CommaSeparatedOptionListConverterTest.java b/src/test/java/com/google/devtools/common/options/CommaSeparatedOptionListConverterTest.java
index 25651ecbfc0ed4..78fe9c0089cc8d 100644
--- a/src/test/java/com/google/devtools/common/options/CommaSeparatedOptionListConverterTest.java
+++ b/src/test/java/com/google/devtools/common/options/CommaSeparatedOptionListConverterTest.java
@@ -14,9 +14,8 @@
 package com.google.devtools.common.options;
 
 import static com.google.common.truth.Truth.assertThat;
-import static org.junit.Assert.assertThrows;
 
-import java.util.List;
+import com.google.common.collect.ImmutableList;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -24,8 +23,7 @@
 /** A test for {@link Converters.CommaSeparatedOptionListConverter}. */
 @RunWith(JUnit4.class)
 public class CommaSeparatedOptionListConverterTest {
-
-  private Converter<List<String>> converter =
+  private final Converter<ImmutableList<String>> converter =
       new Converters.CommaSeparatedOptionListConverter();
 
   @Test
@@ -65,13 +63,4 @@ public void multiWords() throws Exception {
   public void spaceIsIgnored() throws Exception {
     assertThat(converter.convert("one two three")).containsExactly("one two three");
   }
-
-  @Test
-  public void valueisUnmodifiable() throws Exception {
-    assertThrows(
-        "could modify value",
-        UnsupportedOperationException.class,
-        () -> converter.convert("value").add("other"));
-  }
-
 }