The Issue is posted in the NullGuard.Fody repository with nr. 598.

Problem

When run with NullGuard the record below named FooRecordWithInit returns a System.InvalidProgramException: 'Common Language Runtime detected an invalid program.' . With the setup for NullGuard Mode="NullableReferenceTypes" and IncludeDebugAssert="false"/>.

public record FooRecordWithInit
{
    private int _id;
    public int Id
    {
        init => _id = value;  //< ERROR: Common Language Runtime detected an invalid program.
    }
}

The underlying issue seems to be the invalid IL weaved by Fody as seen here:

public int Id
{
  init
  {
    _id = value;
    if ((int)/*Error near IL_0007: Stack underflow*/ == 0)
    {
    throw new InvalidOperationException("[NullGuard] Return value of method 'System.Void modreq(System.Runtime.CompilerServices.IsExternalInit) Bug.FooRecordWithInit::set_Id(System.Int32)' is null.");
    }
  }
}

In other situations a record works fine with a setter instead of init:

public record FooRecordWithSet
{
    private int _id;

    public int Id
    {
        set => _id = value; //< Works
    }
}

Reproduce the issue

A minimal Solution to reproduce the issue can be found here.

1. Clone the repository.
2. Build the project.
3. Use ILSpy to view the output.

Configuration

Operating System: Windows 11 Enterprise 64-Bit (10.0, Build 22621)

Framework Version: .NET 6

Packages: Fody 6.6.4, NullGuard.Fody 3.1.0

x64