Index: test/testIO.rb
===================================================================
--- test/testIO.rb	(revision 3824)
+++ test/testIO.rb	(working copy)
@@ -7,6 +7,7 @@
 @file3 = "Test3IO_tmp"
 
 test_exception(ArgumentError) { IO.new }
+test_exception(StandardError) { IO.new(123) }
 test_exception(TypeError) { IO.new "FROGGER" }
 test_exception(TypeError) { IO.foreach 3 }
 
Index: src/org/jruby/RubyIO.java
===================================================================
--- src/org/jruby/RubyIO.java	(revision 3824)
+++ src/org/jruby/RubyIO.java	(working copy)
@@ -40,6 +40,7 @@
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.lang.ref.WeakReference;
+import java.lang.ref.Reference;
 import java.nio.channels.Channel;
 import java.nio.channels.Pipe;
 import java.nio.channels.SelectableChannel;
@@ -163,7 +164,11 @@
     }
     
     public IOHandler getIOHandlerByFileno(int aFileno) {
-        return (IOHandler) ((WeakReference) getRuntime().getIoHandlers().get(new Integer(aFileno))).get();
+        Reference reference = ((Reference) getRuntime().getIoHandlers().get(new Integer(aFileno)));
+        if (reference == null) {
+            return null;
+        }
+        return (IOHandler) reference.get();
     }
     
     // FIXME can't use static; would interfere with other runtimes in the same JVM