diff --git a/logic.py b/logic.py
index 338e5aac2..75c461e8f 100644
--- a/logic.py
+++ b/logic.py
@@ -796,6 +796,8 @@ def is_variable(x):
 def unify_var(var, x, s):
     if var in s:
         return unify(s[var], x, s)
+    elif x in s:
+        return unify(var, s[x], s)
     elif occur_check(var, x, s):
         return None
     else: