diff --git a/dd/mdd.py b/dd/mdd.py
index b15fea54..fc9a6d92 100644
--- a/dd/mdd.py
+++ b/dd/mdd.py
@@ -296,9 +296,9 @@ def collect_garbage(self, roots=None):
 
     def to_expr(self, u):
         if u == 1:
-            return u
+            return str(u)
         elif u == -1:
-            return 0
+            return '0'
         t = self._succ[abs(u)]
         i = t[0]
         nodes = t[1:]