Getting zero value for ITypeSymbol

I am writing some tests and I need to get the ITypeSymbol. This is how I do it:

    private async Task<ITypeSymbol> GetTypeSymbol(string file, TextSpan span)
    {
        var code = File.ReadAllText(file);
        var tree = CSharpSyntaxTree.ParseText(code);

        var compilation = CSharpCompilation.Create(
            this.GetType().Assembly.GetName().Name,
            syntaxTrees: new[] { tree },
            references: new[]
            {
                MetadataReference.CreateFromAssembly(typeof(object).Assembly))
            });

        var model = compilation.GetSemanticModel(tree);
        var root = await tree.GetRootAsync().ConfigureAwait(false);
        return model.GetTypeInfo(root.FindNode(span)).Type;
    }

      

        
        
        
      

    

For some reason, the Type property on the last line is always null, even if I pass code from a file that looks like this:

public class AClass { }

      

        
        
        
      

    

To be clear, the file reads fine, it parses correctly, the compilation is done, I can get the model, and FindNode () will find the correct ClassDeclarationSyntax node I'm looking for. GetTypeInfo () returns, but both type and ConvertedType are null. I kind of expected it to really matter. Is my expectation correct? If so, what am I doing wrong to make the Type property null?