diff --git a/tools/generator/SymbolTable.cs b/tools/generator/SymbolTable.cs
index 6b567bdf0..0c826c9e4 100644
--- a/tools/generator/SymbolTable.cs
+++ b/tools/generator/SymbolTable.cs
@@ -135,6 +135,9 @@ static bool ShouldAddType (string key)
 
 		public static void AddType (string key, ISymbol symbol)
 		{
+			if (!ShouldAddType (key))
+				return;
+
 			List<ISymbol> values;
 			if (!symbols.TryGetValue (key, out values)) {
 				symbols.Add (key, new List<ISymbol> { symbol });