diff --git a/beets/util/pipeline.py b/beets/util/pipeline.py
index b81db3c7f..f9d3b27e9 100644
--- a/beets/util/pipeline.py
+++ b/beets/util/pipeline.py
@@ -304,11 +304,11 @@ class Pipeline(object):
             raise ValueError('pipeline must have at least two stages')
         self.stages = []
         for stage in stages:
-            if isinstance(stage, types.GeneratorType):
+            if isinstance(stage, (list, tuple)):
+                self.stages.append(stage)
+            else:
                 # Default to one thread per stage.
                 self.stages.append((stage,))
-            else:
-                self.stages.append(stage)
 
     def run_sequential(self):
         """Run the pipeline sequentially in the current thread. The