CS8785 when generating code for class in global namespace

[deerchao]

Program.cs

// See https://aka.ms/new-console-template for more information
using OneOf;

Console.WriteLine("Hello, World!");

[GenerateOneOf]
public partial class StringOrNumber : OneOfBase<string, int> { }

Error:

Generator 'OneOfGenerator' failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'ArgumentException' with message 'The hintName '<global namespace>_StringOrNumber.g.cs' contains an invalid character '<' at position 0.