diff --git a/src/TensorFlowNET.Core/Graphs/DefaultGraphStack.cs b/src/TensorFlowNET.Core/Graphs/DefaultGraphStack.cs
index 6c9f6b18..42612d4e 100644
--- a/src/TensorFlowNET.Core/Graphs/DefaultGraphStack.cs
+++ b/src/TensorFlowNET.Core/Graphs/DefaultGraphStack.cs
@@ -14,49 +14,61 @@
    limitations under the License.
 ******************************************************************************/
 
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using static Tensorflow.Binding;
 
 namespace Tensorflow
 {
+    /// <summary>
+    ///     Serves as a stack for determining current default graph.
+    /// </summary>
     public class DefaultGraphStack
     {
-        List<StackModel> stack = new List<StackModel>();
+        private readonly List<StackModel> _stack = new List<StackModel>();
 
         public void set_controller(Graph @default)
         {
-            if (!stack.Exists(x => x.Graph == @default))
-                stack.Add(new StackModel { Graph = @default, IsDefault = true });
+            if (!_stack.Exists(x => x.Graph == @default))
+                _stack.Add(new StackModel {Graph = @default, IsDefault = true});
 
-            foreach (var s in stack)
+            foreach (var s in _stack)
                 s.IsDefault = s.Graph == @default;
         }
 
         public Graph get_controller()
         {
-            if (stack.Count(x => x.IsDefault) == 0)
-                stack.Add(new StackModel { Graph = tf.Graph(), IsDefault = true });
+            if (_stack.Count(x => x.IsDefault) == 0)
+                _stack.Add(new StackModel {Graph = tf.Graph(), IsDefault = true});
+            for (var i = _stack.Count - 1; i >= 0; i--)
+            {
+                var x = _stack[i];
+                if (x.IsDefault)
+                    return x.Graph;
+            }
 
-            return stack.Last(x => x.IsDefault).Graph;
+            throw new TensorflowException("Unable to find a default graph");
         }
 
         public bool remove(Graph g)
         {
-            var sm = stack.FirstOrDefault(x => x.Graph == g);
-            if (sm == null) return false;
-            return stack.Remove(sm);
+            if (_stack.Count == 0)
+                return false;
+
+            var sm = _stack.Find(model => model.Graph == g);
+            return sm != null && _stack.Remove(sm);
         }
 
         public void reset()
         {
-            stack.Clear();
+            _stack.Clear();
         }
-    }
 
-    public class StackModel
-    {
-        public Graph Graph { get; set; }
-        public bool IsDefault { get; set; }
+        private class StackModel
+        {
+            public Graph Graph { get; set; }
+            public bool IsDefault { get; set; }
+        }
     }
-}
+}
\ No newline at end of file