diff --git a/ZeroLevel.UnitTests/SemanticTests.cs b/ZeroLevel.UnitTests/SemanticTests.cs
index ba16b1d..b5d75da 100644
--- a/ZeroLevel.UnitTests/SemanticTests.cs
+++ b/ZeroLevel.UnitTests/SemanticTests.cs
@@ -29,6 +29,16 @@ namespace ZeroLevel.UnitTests
             {
                 Assert.True(string.CompareOrdinal(test[i], terms[i]) == 0);
             }
+
+            Assert.False(WordTokenizer.Tokenize(string.Empty).Any());
+            Assert.False(WordTokenizer.Tokenize(null).Any());
+            Assert.False(WordTokenizer.Tokenize(" ").Any());
+            Assert.False(WordTokenizer.Tokenize("                        ").Any());
+            Assert.False(WordTokenizer.Tokenize(" 1 ").Any());
+            Assert.False(WordTokenizer.Tokenize("1 1").Any());
+            Assert.False(WordTokenizer.Tokenize(" 1 1 ").Any());
+            Assert.False(WordTokenizer.Tokenize(" 12aa 1a3 ").Any());
+            Assert.False(WordTokenizer.Tokenize(" __a1 _a1 ").Any());
         }
     }
 }
diff --git a/ZeroLevel/Services/Semantic/Trie.cs b/ZeroLevel/Services/Semantic/Trie.cs
index 31effa7..d13367a 100644
--- a/ZeroLevel/Services/Semantic/Trie.cs
+++ b/ZeroLevel/Services/Semantic/Trie.cs
@@ -15,7 +15,7 @@ namespace ZeroLevel.Services.Semantic
             public char? Key; // setted only with rebuild index
             public uint? Value;
             public TrieNode Parent;
-            public ConcurrentDictionary<char, TrieNode> Children;
+            public Dictionary<char, TrieNode> Children;
 
             public TrieNode() { }
             public TrieNode(TrieNode parent) { Parent = parent; }
@@ -29,7 +29,7 @@ namespace ZeroLevel.Services.Semantic
                 {
                     this.Value = null;
                 }
-                this.Children = reader.ReadDictionaryAsConcurrent<char, TrieNode>();
+                this.Children = reader.ReadDictionary<char, TrieNode>();
             }
 
             public void Serialize(IBinaryWriter writer)
@@ -52,7 +52,7 @@ namespace ZeroLevel.Services.Semantic
                     writer.WriteDictionary<char, TrieNode>(this.Children);
                 }
             }
-            internal TrieNode Append(string word, int index, bool reverse)
+            internal TrieNode Append(string word, int index)
             {
                 if (word.Length == index)
                 {
@@ -65,13 +65,13 @@ namespace ZeroLevel.Services.Semantic
                 {
                     if (this.Children == null)
                     {
-                        this.Children = new ConcurrentDictionary<char, TrieNode>();
+                        this.Children = new Dictionary<char, TrieNode>();
                     }
                     if (!this.Children.ContainsKey(word[index]))
                     {
-                        this.Children.TryAdd(word[index], new TrieNode(this));
+                        this.Children.Add(word[index], new TrieNode(this));
                     }
-                    return Children[word[index]].Append(word, index + 1, reverse);
+                    return Children[word[index]].Append(word, index + 1);
                 }
                 return null;
             }
@@ -154,7 +154,7 @@ namespace ZeroLevel.Services.Semantic
         public void Append(string word)
         {
             if (word.Length == 0) return;
-            var node = _root.Append(word, 0, _use_reverse_index);
+            var node = _root.Append(word, 0);
             if (node != null)
             {
                 node.Value = (uint)Interlocked.Increment(ref _word_index);
diff --git a/ZeroLevel/Services/Semantic/WordTokenizer.cs b/ZeroLevel/Services/Semantic/WordTokenizer.cs
index 17490d2..b5fa5b0 100644
--- a/ZeroLevel/Services/Semantic/WordTokenizer.cs
+++ b/ZeroLevel/Services/Semantic/WordTokenizer.cs
@@ -15,7 +15,7 @@ namespace ZeroLevel.Services.Semantic
             var buffer = _pool.Allocate();
             try
             {
-                for (int i = 0; i < text.Length; i++)
+                for (int i = 0; i < text?.Length; i++)
                 {
                     if (first && Char.IsLetter(text[i]))
                     {