diff --git a/sdks/python/apache_beam/transforms/util.py b/sdks/python/apache_beam/transforms/util.py
index 5f99d6805469..868dbe1361b8 100644
--- a/sdks/python/apache_beam/transforms/util.py
+++ b/sdks/python/apache_beam/transforms/util.py
@@ -234,7 +234,8 @@ def expand(self, pcolls):
     for pcoll in pcolls.values():
       self._check_pcollection(pcoll)
       if self.pipeline:
-        assert pcoll.pipeline == self.pipeline
+        assert pcoll.pipeline == self.pipeline, (
+            'All input PCollections must belong to the same pipeline.')
 
     tags = list(pcolls.keys())