diff --git a/andes/models/base.py b/andes/models/base.py
index 586399a70..c48e43020 100644
--- a/andes/models/base.py
+++ b/andes/models/base.py
@@ -424,15 +424,21 @@ def get_field(self, field, idx=None, astype=None):
         # =================================================================
 
         uid = self.get_uid(idx)
-        if not astype:
-            astype = type(self.__dict__[field])
 
-        ret = matrix(self.__dict__[field])[uid]
+        field_data = self.__dict__[field]
 
-        if isinstance(idx, (float, int, str)):
-            return ret
-        else:
-            return astype(ret)
+        if isinstance(field_data, matrix):
+            ret = field_data[uid]
+        elif isinstance(field_data, list):
+            if isinstance(idx, (float, int, str)):
+                ret = field_data[uid]
+            else:
+                ret = [field_data[x] for x in uid]
+
+        if astype is not None:
+            ret = astype(ret)
+
+        return ret
 
     def _alloc(self):
         """
diff --git a/andes/models/zone.py b/andes/models/zone.py
index 3bc0c60d7..8d360c4a5 100644
--- a/andes/models/zone.py
+++ b/andes/models/zone.py
@@ -208,6 +208,7 @@ class Area(Zone):
     def __init__(self, system, name):
         super().__init__(system, name)
         self._name = 'Area'
+        self._group = 'Collection'
         self._init()
 
     def setup(self):
@@ -221,6 +222,7 @@ class Region(Zone):
     def __init__(self, system, name):
         super().__init__(system, name)
         self._name = 'Region'
+        self._group = 'Balancing'
         self._params.extend(['Ptol', 'slack'])
         self._descr.update({
             'Ptol': 'Total transfer capacity',
diff --git a/andes/system.py b/andes/system.py
index cd912d6e2..3bca53ca8 100644
--- a/andes/system.py
+++ b/andes/system.py
@@ -625,6 +625,7 @@ def __init__(self, system, name):
         self.name = name
         self.all_models = []
         self._idx_model = {}
+        self._idx = []
 
     def register_model(self, model):
         """
@@ -650,9 +651,14 @@ def register_element(self, model, idx):
         if idx is None:
             idx = model + '_' + str(len(self._idx_model))
 
-        assert idx not in self._idx_model.values()
+        # TODO: `in` a list test is slow for large lists. Consider `bisect`
+        if idx in self._idx:
+            raise IndexError("Model {} idx {} already exist in model {}".
+                             format(model, idx, self._idx_model[idx]))
 
         self._idx_model[idx] = model
+        self._idx.append(idx)
+
         return idx
 
     def get_field(self, field, idx):