diff --git a/OptionStudy.UnitApp/YamlStream.cs b/OptionStudy.UnitApp/YamlStream.cs
index 498d83e..9268098 100644
--- a/OptionStudy.UnitApp/YamlStream.cs
+++ b/OptionStudy.UnitApp/YamlStream.cs
@@ -49,7 +49,7 @@ namespace OptionStudy.UnitApp
         public override void Load(Stream stream)
         {
             var parser = new YamlFileParser();
-            Data =parser.Parse(stream);
+            Data = parser?.Parse(stream)??new Dictionary<string, string?>();
         }
     }
 
@@ -73,11 +73,11 @@ namespace OptionStudy.UnitApp
     /// </summary>
     public class YamlFileParser
     {
-        private readonly IDictionary<string, string> _data = new SortedDictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        private readonly IDictionary<string, string?> _data = new SortedDictionary<string, string?>(StringComparer.OrdinalIgnoreCase);
         private readonly Stack<string> _context = new Stack<string>();
-        private string _currentPath;
+        private string? _currentPath;
 
-        public IDictionary<string, string> Parse(Stream input)
+        public IDictionary<string, string?> Parse(Stream input)
         {
             _data.Clear();
             _context.Clear();
@@ -99,7 +99,7 @@ namespace OptionStudy.UnitApp
 
         private void VisitYamlNodePair(KeyValuePair<YamlNode, YamlNode> yamlNodePair)
         {
-            var context = ((YamlScalarNode)yamlNodePair.Key).Value;
+            var context = ((YamlScalarNode)yamlNodePair.Key).Value??string.Empty;
             VisitYamlNode(context, yamlNodePair.Value);
         }
 
@@ -123,7 +123,7 @@ namespace OptionStudy.UnitApp
         {
             //a node with a single 1-1 mapping 
             EnterContext(context);
-            var currentKey = _currentPath;
+            var currentKey = _currentPath??string.Empty;
 
             if (_data.ContainsKey(currentKey))
             {