diff --git a/lib/perfectqueue/backend/rdb_compat.rb b/lib/perfectqueue/backend/rdb_compat.rb
index aace128..feee196 100644
--- a/lib/perfectqueue/backend/rdb_compat.rb
+++ b/lib/perfectqueue/backend/rdb_compat.rb
@@ -259,15 +259,16 @@ def acquire(alive_time, max_acquire, options)
             return nil
           end
 
-          sql = "UPDATE `#{@table}` SET timeout=? WHERE id IN ("
-          params = [sql, next_timeout]
+          sql = "UPDATE `#{@table}` SET timeout=? WHERE timeout <= ? AND id IN ("
+          params = [sql, next_timeout, now]
           tasks.each {|t| params << t.key }
           sql << (1..tasks.size).map { '?' }.join(',')
           sql << ") AND created_at IS NOT NULL"
 
           n = @db[*params].update
           if n != tasks.size
-            # TODO table lock doesn't work. error?
+            # NOTE table lock doesn't work. error!
+            return nil
           end
 
           @cleanup_interval_count -= 1
diff --git a/spec/rdb_compat_backend_spec.rb b/spec/rdb_compat_backend_spec.rb
index 0dfd93a..3bfb8cd 100644
--- a/spec/rdb_compat_backend_spec.rb
+++ b/spec/rdb_compat_backend_spec.rb
@@ -228,6 +228,26 @@
         expect(ary[1]).to be_an_instance_of(AcquiredTask)
       end
     end
+    context 'stole a task' do
+      let :t0 do now - 100 end
+      before do
+        db.submit('key1', 'test1', nil, {now: t0})
+        db.submit('key2', 'test2', nil, {now: t0})
+        db.submit('key3', 'test3', nil, {now: t0})
+      end
+      it 'returns nil' do
+        # hook and steal a task
+        mock = double('prefetch_break_types')
+        db.instance_variable_set(:@prefetch_break_types, mock)
+        allow(mock).to receive(:include?) do
+          db.db['UPDATE `test_queues` SET timeout=? WHERE id=?', now+300, 'key2'].update
+          false
+        end
+
+        ary = db.acquire(alive_time, max_acquire, {})
+        expect(ary).to be_nil
+      end
+    end
   end
 
   context '#cancel_request' do