Index: src/org/jruby/RubyThread.java
===================================================================
--- src/org/jruby/RubyThread.java	(revision 4841)
+++ src/org/jruby/RubyThread.java	(working copy)
@@ -341,7 +341,7 @@
     }
 
     @JRubyMethod(name = "join", optional = 1)
-    public RubyThread join(IRubyObject[] args) {
+    public IRubyObject join(IRubyObject[] args) {
         long timeoutMillis = 0;
         if (args.length > 0) {
             if (args.length > 1) {
@@ -351,7 +351,10 @@
             // than or equal to zero returns immediately; returns nil
             timeoutMillis = (long)(1000.0D * args[0].convertToFloat().getValue());
             if (timeoutMillis <= 0) {
-                return null;
+                if (threadImpl.isAlive()) {
+                    return getRuntime().getNil();
+                }
+                return this;
             }
         }
         if (isCurrent()) {
@@ -372,7 +375,10 @@
         if (exitingException != null) {
             throw exitingException;
         }
-        return null;
+        if (threadImpl.isAlive()) {
+            return getRuntime().getNil();
+        }
+        return this;
     }
 
     @JRubyMethod(name = "value")
Index: build.xml
===================================================================
--- build.xml	(revision 4841)
+++ build.xml	(working copy)
@@ -413,6 +413,7 @@
 ${build.dir}/rubinius-spec/core/proc_spec.rb
 ${build.dir}/rubinius-spec/core/process_spec.rb
 ${build.dir}/rubinius-spec/core/regexp_spec.rb
+${build.dir}/rubinius-spec/core/thread_spec.rb
 ${build.dir}/rubinius-spec/core/true_spec.rb
 ${build.dir}/rubinius-spec/language/array_spec.rb
 ${build.dir}/rubinius-spec/language/assignment_spec.rb
@@ -446,6 +447,7 @@
 ${build.dir}/rubinius-spec/core/proc_spec.rb
 ${build.dir}/rubinius-spec/core/process_spec.rb
 ${build.dir}/rubinius-spec/core/regexp_spec.rb
+${build.dir}/rubinius-spec/core/thread_spec.rb
 ${build.dir}/rubinius-spec/core/true_spec.rb
 ${build.dir}/rubinius-spec/language/array_spec.rb
 ${build.dir}/rubinius-spec/language/assignment_spec.rb
@@ -484,6 +486,7 @@
 test/externals/rubinius/spec/core/regexp_spec.rb
 test/externals/rubinius/spec/core/struct_spec.rb
 test/externals/rubinius/spec/core/symbol_spec.rb
+test/externals/rubinius/spec/core/thread_spec.rb
 test/externals/rubinius/spec/core/true_spec.rb
 test/externals/rubinius/spec/language/assignment_spec.rb
 test/externals/rubinius/spec/language/class_spec.rb