iOS crash: malloc: *** error for object XXXX: pointer being freed was not allocated

I am using https://github.com/miyabi/unity-swift plugin to call a Swift audio conversion method. This is working fine, except that when the Swift code returns it's string, the Unity generated C++ code throws the
error:

malloc: *** error for object XXXX: pointer being freed was not allocated

from the line (in Bulk_Assembly-CSharp-firstpass_0.cpp):

il2cpp_codegen_marshal_free(returnValue);

If I comment out that line the error is no longer thrown and there does not seem to be any leak. My question is, can I flag my code somehow before building so that this line is automatically omitted? Otherwise I have to manually find it to comment it out as the audio conversion is a rarely used functionality.

Here is the C# code:

using System.Runtime.InteropServices;



public class AudioConverter {

    #if UNITY_IOS && !UNITY_EDITOR

    [DllImport("__Internal")]

    private static extern string _swift_TryConvertAudio(string message);

    #endif



    // Use this method to call AudioConverter.swiftMethod() in AudioConverter.swift

    // from other C# classes.

    public static string TryConvertAudio(string dirPath) {

        #if UNITY_IOS && !UNITY_EDITOR

        return _swift_TryConvertAudio(dirPath);

        #endif

        return "[\"Platform Unsupported\"]";

    }

}

Here is a screenshot of the crash in action, for stack examination.