diff --git a/src/Compilers/Core/Portable/Syntax/SyntaxListBuilder`1.cs b/src/Compilers/Core/Portable/Syntax/SyntaxListBuilder`1.cs
index 9ba2c897ffd59023a2345b1c5600607b1754c54c..6348741935403588ae6d1e2865b8549d4195e386 100644
--- a/src/Compilers/Core/Portable/Syntax/SyntaxListBuilder`1.cs
+++ b/src/Compilers/Core/Portable/Syntax/SyntaxListBuilder`1.cs
@@ -6,7 +6,7 @@ namespace Microsoft.CodeAnalysis.Syntax
 {
     internal struct SyntaxListBuilder<TNode> where TNode : SyntaxNode
     {
-        private readonly SyntaxListBuilder _builder;
+        private readonly SyntaxListBuilder? _builder;
 
         public SyntaxListBuilder(int size)
             : this(new SyntaxListBuilder(size))
@@ -18,7 +18,7 @@ public static SyntaxListBuilder<TNode> Create()
             return new SyntaxListBuilder<TNode>(8);
         }
 
-        internal SyntaxListBuilder(SyntaxListBuilder builder)
+        internal SyntaxListBuilder(SyntaxListBuilder? builder)
         {
             _builder = builder;
         }
@@ -35,39 +35,39 @@ public int Count
         {
             get
             {
-                return _builder.Count;
+                return _builder!.Count;
             }
         }
 
         public void Clear()
         {
-            _builder.Clear();
+            _builder!.Clear();
         }
 
         public SyntaxListBuilder<TNode> Add(TNode node)
         {
-            _builder.Add(node);
+            _builder!.Add(node);
             return this;
         }
 
         public void AddRange(TNode[] items, int offset, int length)
         {
-            _builder.AddRange(items, offset, length);
+            _builder!.AddRange(items, offset, length);
         }
 
         public void AddRange(SyntaxList<TNode> nodes)
         {
-            _builder.AddRange(nodes);
+            _builder!.AddRange(nodes);
         }
 
         public void AddRange(SyntaxList<TNode> nodes, int offset, int length)
         {
-            _builder.AddRange(nodes, offset, length);
+            _builder!.AddRange(nodes, offset, length);
         }
 
         public bool Any(int kind)
         {
-            return _builder.Any(kind);
+            return _builder!.Any(kind);
         }
 
         public SyntaxList<TNode> ToList()
@@ -75,7 +75,7 @@ public SyntaxList<TNode> ToList()
             return _builder.ToList();
         }
 
-        public static implicit operator SyntaxListBuilder(SyntaxListBuilder<TNode> builder)
+        public static implicit operator SyntaxListBuilder?(SyntaxListBuilder<TNode> builder)
         {
             return builder._builder;
         }
@@ -87,7 +87,7 @@ public SyntaxList<TNode> ToList()
                 return builder.ToList();
             }
 
-            return default(SyntaxList<TNode>);
+            return default;
         }
     }
 }