diff --git a/pymc3/distributions/transforms.py b/pymc3/distributions/transforms.py
index <HASH>..<HASH> 100644
--- a/pymc3/distributions/transforms.py
+++ b/pymc3/distributions/transforms.py
@@ -128,7 +128,7 @@ class SumTo1(Transform):
     name = "sumto1"
 
     def backward(self, y):
-        return T.concatenate([y, 1-sum(y, keepdims=True)])
+        return T.concatenate([y, 1 - T.sum(y, keepdims=True)])
 
     def forward(self, x):
         return x[:-1]
BUG use ``sum`` from Theano in ``SumTo1``