diff --git a/core/src/main/java/org/jruby/RubyThreadGroup.java b/core/src/main/java/org/jruby/RubyThreadGroup.java
index 652c4939b83..9518e013d41 100644
--- a/core/src/main/java/org/jruby/RubyThreadGroup.java
+++ b/core/src/main/java/org/jruby/RubyThreadGroup.java
@@ -95,12 +95,11 @@ public IRubyObject add(ThreadContext context, IRubyObject rubyThread, Block bloc
             }
         }
 
-        // always set group when added explicitly
-        thread.setThreadGroup(this);
-
         // we only add live threads
         if (thread.alive_p(context).isTrue()) {
             addDirectly(thread);
+        } else {
+            thread.setThreadGroup(this);
         }
         
         return this;