diff --git a/tests/pipeline/test_engine.py b/tests/pipeline/test_engine.py
index 75b3387f..b3190d48 100644
--- a/tests/pipeline/test_engine.py
+++ b/tests/pipeline/test_engine.py
@@ -40,6 +40,7 @@ from zipline.assets.synthetic import make_rotating_equity_info
 from zipline.errors import NoFurtherDataError
 from zipline.lib.adjustment import MULTIPLY
 from zipline.lib.labelarray import LabelArray
+from zipline.lib.rank import masked_rankdata_2d
 from zipline.pipeline import CustomFactor, Pipeline
 from zipline.pipeline.data import Column, DataSet, USEquityPricing
 from zipline.pipeline.data.testing import TestingDataSet
diff --git a/zipline/pipeline/expression.py b/zipline/pipeline/expression.py
index d99ab341..941f058e 100644
--- a/zipline/pipeline/expression.py
+++ b/zipline/pipeline/expression.py
@@ -187,6 +187,7 @@ class NumericalExpression(ComputableTerm):
             inputs=binds,
             expr=expr,
             dtype=dtype,
+            window_safe=all(t.window_safe for t in binds),
         )
 
     def _init(self, expr, *args, **kwargs):