async arrow functions fail on target es2018

Hey there I cannot seem to get async arrow functions of a class to work

    class LogClass {
      public error = async (_: string) => null;
      public warn = async (_: string) => null;
    }
    const fakeLogger = mock(LogClass);
    when(fakeLogger.error(anything())).thenResolve(null) // fakeLogger.error is not a function
    // nor does
    when(fakeLogger.warn(anything())).thenReturn(Promise.resolve(null)); //fakeLogger.warn is not a function

However

    class LogClass {
      public error = (_: string) => null;
      public async warn (_: string) { return null };
    }
    const fakeLogger = mock(LogClass);
    when(fakeLogger.error(anything())).thenReturn(null);
    when(fakeLogger.warn(anything())).thenResolve();

works. I would like it if I could use async arrow functions

[dice14u]

Figured out the difference in my tsconfig.json

{
  "compilerOptions": {
    "sourceMap": true,
    "target": "es2016",
    "lib": ["esnext.asynciterable", "es7", "dom"]
  },
}

works

{
  "compilerOptions": {
    "sourceMap": true,
    "target": "es2018",
    "lib": ["esnext.asynciterable", "es7", "dom"]
  },
}

fails

[dice14u]

@NagRock ^ ?