diff --git a/orchard_pallas.py b/orchard_pallas.py
index 109fa1b..40c24da 100644
--- a/orchard_pallas.py
+++ b/orchard_pallas.py
@@ -182,6 +182,13 @@ class Point(object):
             else:
                 return self.double()
 
+    def checked_incomplete_add(self, a):
+        assert self != a
+        assert self != -a
+        assert self != Point.identity()
+        assert self != Point.identity()
+        return self + a
+
     def __sub__(self, a):
         return (-a) + self
 
diff --git a/orchard_sinsemilla.py b/orchard_sinsemilla.py
index 054d4a0..e76224d 100755
--- a/orchard_sinsemilla.py
+++ b/orchard_sinsemilla.py
@@ -155,11 +155,11 @@ def sinsemilla_hash_to_point(d, m):
     n = cldiv(m.len, SINSEMILLA_K)
     m = pad(n, m)
     acc = group_hash(b"z.cash:SinsemillaQ", d)
-    #print("acc", acc)
 
     for m_i in m:
-        acc = acc + group_hash(b"z.cash:SinsemillaS", i2leosp(32, m_i)) + acc
-        #print("acc", acc)
+        acc = acc.checked_incomplete_add(
+            group_hash(b"z.cash:SinsemillaS", i2leosp(32, m_i))
+        ).checked_incomplete_add(acc)
 
     return acc