diff --git lib/pstore.rb lib/pstore.rb
index 3114eb5..9fb0249 100644
--- lib/pstore.rb
+++ lib/pstore.rb
@@ -341,6 +341,8 @@ class PStore
       end
     end
     value
+  rescue ThreadError
+    raise PStore::Error, "nested transaction"
   end
 
   private
diff --git test/test_pstore.rb test/test_pstore.rb
index 32d79ea..b6f7925 100644
--- test/test_pstore.rb
+++ test/test_pstore.rb
@@ -100,4 +100,14 @@ class PStoreTest < Test::Unit::TestCase
       File.unlink("pstore.tmp2.#{Process.pid}") rescue nil
     end
   end
+  
+  def test_nested_transaction_raises_error
+    assert_raise(PStore::Error) do
+      @pstore.transaction { @pstore.transaction { } }
+    end
+    pstore = PStore.new("pstore.tmp2.#{Process.pid}", true)
+    assert_raise(PStore::Error) do
+      pstore.transaction { pstore.transaction { } }
+    end
+  end
 end