diff --git a/src/pyhf/workspace.py b/src/pyhf/workspace.py
index 1dd3ee47c8..08b285d25e 100644
--- a/src/pyhf/workspace.py
+++ b/src/pyhf/workspace.py
@@ -18,6 +18,8 @@
 from pyhf import exceptions, schema
 from pyhf.mixins import _ChannelSummaryMixin
 from pyhf.pdf import Model
+import functools
+import operator
 
 log = logging.getLogger(__name__)
 
@@ -465,8 +467,8 @@ def data(self, model, include_auxdata=True):
 
         """
         try:
-            observed_data = sum(
-                (self.observations[c] for c in model.config.channels), []
+            observed_data = functools.reduce(
+                operator.iadd, (self.observations[c] for c in model.config.channels), []
             )
         except KeyError:
             log.error(