diff --git a/src/hyperactive/optimizers/constraint.py b/src/hyperactive/optimizers/constraint.py
index e170970e..f6d28f93 100644
--- a/src/hyperactive/optimizers/constraint.py
+++ b/src/hyperactive/optimizers/constraint.py
@@ -5,9 +5,9 @@
 
 def gfo2hyper(search_space, para):
     values_dict = {}
-    for _, key in enumerate(search_space.keys()):
+    for key, values in search_space.items():
         pos_ = int(para[key])
-        values_dict[key] = search_space[key][pos_]
+        values_dict[key] = values[pos_]
 
     return values_dict
 
diff --git a/src/hyperactive/search_space.py b/src/hyperactive/search_space.py
index 101201d8..cb90e092 100644
--- a/src/hyperactive/search_space.py
+++ b/src/hyperactive/search_space.py
@@ -18,6 +18,9 @@ def keys(self):
     def values(self):
         return self.search_space.values()
 
+    def items(self):
+        return self.search_space.items()
+
 
 class SearchSpace(DictClass):
     def __init__(self, search_space):