diff --git a/lib/thread.rb b/lib/thread.rb
index 09f2d58..524db69 100644
--- a/lib/thread.rb
+++ b/lib/thread.rb
@@ -269,6 +269,7 @@ class SizedQueue < Queue
   # Sets the maximum size of the queue.
   #
   def max=(max)
+    raise ArgumentError, "queue size must be positive" unless max > 0
     diff = nil
     @mutex.synchronize {
       if max <= @max