diff --git a/core/src/main/java/org/jruby/internal/runtime/methods/InvocationMethodFactory.java b/core/src/main/java/org/jruby/internal/runtime/methods/InvocationMethodFactory.java
index 82f90ba5f44..81f6c7b815d 100644
--- a/core/src/main/java/org/jruby/internal/runtime/methods/InvocationMethodFactory.java
+++ b/core/src/main/java/org/jruby/internal/runtime/methods/InvocationMethodFactory.java
@@ -635,9 +635,8 @@ private static void loadReceiver(String typePath, JavaMethodDescriptor desc, Ski
     private Class tryClass(String name, String path, Class targetClass, Class expectedSuperclass) {
         final Class c;
         try {
-            URL resource = classLoader.findResource(path + ".class");
-            if (resource == null) {
-                if (DEBUG) LOG.debug("could not find class file for " + name);
+            if (!classLoader.hasClass(name)) {
+                if (DEBUG) System.err.println("could not find class file for " + name);
                 seenUndefinedClasses = true;
                 return null;
             }
diff --git a/core/src/main/java/org/jruby/util/ClassDefiningJRubyClassLoader.java b/core/src/main/java/org/jruby/util/ClassDefiningJRubyClassLoader.java
index 71cdc09d33b..164bcc97ca0 100644
--- a/core/src/main/java/org/jruby/util/ClassDefiningJRubyClassLoader.java
+++ b/core/src/main/java/org/jruby/util/ClassDefiningJRubyClassLoader.java
@@ -30,11 +30,16 @@
 import java.net.URL;
 import java.net.URLClassLoader;
 import java.security.ProtectionDomain;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentSkipListSet;
 
 public class ClassDefiningJRubyClassLoader extends URLClassLoader implements ClassDefiningClassLoader {
 
     public final static ProtectionDomain DEFAULT_DOMAIN;
 
+    private final Set<String> definedClasses = new ConcurrentSkipListSet<>();
+
     static {
         ProtectionDomain defaultDomain = null;
         try {
@@ -50,10 +55,24 @@ public ClassDefiningJRubyClassLoader(ClassLoader parent) {
     }
 
     public Class<?> defineClass(String name, byte[] bytes) {
-        return super.defineClass(name, bytes, 0, bytes.length, DEFAULT_DOMAIN);
+        Class<?> aClass = super.defineClass(name, bytes, 0, bytes.length, DEFAULT_DOMAIN);
+        definedClasses.add(name);
+        return aClass;
     }
 
     public Class<?> defineClass(String name, byte[] bytes, ProtectionDomain domain) {
-        return super.defineClass(name, bytes, 0, bytes.length, domain);
+        Class<?> aClass = super.defineClass(name, bytes, 0, bytes.length, domain);
+        definedClasses.add(name);
+        return aClass;
+    }
+
+    /**
+     * Return true if the class is loadable in this classloader, false otherwise.
+     *
+     * @param name the class name
+     * @return whether it's loadable
+     */
+    public boolean hasClass(String name) {
+        return definedClasses.contains(name) || super.findResource(name.replace('.', '/') + ".class") != null;
     }
 }