diff --git a/src/Compilers/CSharp/Portable/Symbols/Tuples/TupleTypeSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Tuples/TupleTypeSymbol.cs
index da4dfb7663716..76c90edde22f8 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Tuples/TupleTypeSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Tuples/TupleTypeSymbol.cs
@@ -1051,7 +1051,11 @@ internal SmallDictionary<Symbol, Symbol> UnderlyingDefinitionToMemberMap
             {
                 get
                 {
-                    return _lazyUnderlyingDefinitionToMemberMap ??= computeDefinitionToMemberMap();
+                    if (_lazyUnderlyingDefinitionToMemberMap is null)
+                    {
+                        Interlocked.CompareExchange(ref _lazyUnderlyingDefinitionToMemberMap, computeDefinitionToMemberMap(), null);
+                    }
+                    return _lazyUnderlyingDefinitionToMemberMap;
 
                     SmallDictionary<Symbol, Symbol> computeDefinitionToMemberMap()
                     {