diff --git a/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java b/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
index dd7bffc5..3e26e755 100644
--- a/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
+++ b/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
@@ -30,6 +30,9 @@ public class PluginClassLoader extends URLClassLoader {
     }
 
     protected Class<?> findClass(String name, boolean checkGlobal) throws ClassNotFoundException {
+        if (name.startsWith("org.bukkit.") || name.startsWith("net.minecraft.")) {
+            throw new ClassNotFoundException(name);
+        }
         Class<?> result = classes.get(name);
 
         if (result == null) {