diff --git a/src/Common/ITypeSymbolExtensions.cs b/src/Common/ITypeSymbolExtensions.cs
index 23134f5e..307a8afe 100644
--- a/src/Common/ITypeSymbolExtensions.cs
+++ b/src/Common/ITypeSymbolExtensions.cs
@@ -12,7 +12,7 @@ internal static class ITypeSymbolExtensions
     /// <returns>The type and any inherited types.</returns>
     public static IEnumerable<ITypeSymbol> GetBaseTypesAndThis(this ITypeSymbol type)
     {
-        var current = type;
+        ITypeSymbol? current = type;
         while (current is not null)
         {
             yield return current;