diff --git a/src/com/google/doclava/Stubs.java b/src/com/google/doclava/Stubs.java
index 930d52c..a90ed6e 100644
--- a/src/com/google/doclava/Stubs.java
+++ b/src/com/google/doclava/Stubs.java
@@ -53,7 +53,7 @@
       boolean keepStubComments) {
     // figure out which classes we need
     final HashSet<ClassInfo> notStrippable = new HashSet<ClassInfo>();
-    Collection<ClassInfo> all = Converter.allClasses();
+    Collection<ClassInfo> all = List.copyOf(Converter.allClasses());
     Map<PackageInfo, List<ClassInfo>> allClassesByPackage = null;
     PrintStream apiWriter = null;
     PrintStream dexApiWriter = null;