diff --git a/core/src/main/java/org/jruby/RubyClass.java b/core/src/main/java/org/jruby/RubyClass.java
index 00ff7353765..116fb8b1af1 100644
--- a/core/src/main/java/org/jruby/RubyClass.java
+++ b/core/src/main/java/org/jruby/RubyClass.java
@@ -1087,7 +1087,7 @@ private void concreteSubclasses(Collection<RubyClass> subs) {
             Set<RubyClass> keys = subclasses.keySet();
             for (RubyClass klass: keys) {
                 if (klass.isSingleton()) continue;
-                if (klass.isIncluded()) {
+                if (klass.isIncluded() || klass.isPrepended()) {
                     klass.concreteSubclasses(subs);
                     continue;
                 }