diff --git a/sdks/python/apache_beam/runners/dataflow/dataflow_runner.py b/sdks/python/apache_beam/runners/dataflow/dataflow_runner.py
index 07c5f8809fc7..02be0062296f 100644
--- a/sdks/python/apache_beam/runners/dataflow/dataflow_runner.py
+++ b/sdks/python/apache_beam/runners/dataflow/dataflow_runner.py
@@ -1021,7 +1021,7 @@ def run_ParDo(self, transform_node, options):
     # will be 'None' for main output and '<tag>' for a tagged output.
     outputs = []
 
-    all_output_tags = transform_proto.outputs.keys()
+    all_output_tags = list(transform_proto.outputs.keys())
 
     # Some external transforms require output tags to not be modified.
     # So we randomly select one of the output tags as the main output and