NestJS – ConfigService GET method is undefined when called within onModuleInit()

I’m trying to setup a transporter in the NestJS lifecylce method onModuleInit() with the npm library “Nodemailer”. So that I can easily send emails from other services in my backend, but I’m getting an error that says the get method is undefined on configService. Bear in mind I can successfully retrieve my environment variables from other methods within my MailService class, it’s just the onModuleInit() that’s cause for grief. Is there some OOP principle that I’m not privy to which is my cause for issue? Any help appreciated, thanks in advance!

CODE

import { OnModuleInit } from '@nestjs/common';
import { ConfigService } from '@nestjs/config';
import * as nodemailer from 'nodemailer';
import SMTPTransport from 'nodemailer/lib/smtp-transport';

interface EnvironmentVariables {
  MAIL_USER: string;
  MAIL_PASS: string;
}

export class MailService implements OnModuleInit {
  transporter: nodemailer.Transporter<SMTPTransport.SentMessageInfo>;
  constructor(private configService: ConfigService<EnvironmentVariables>) {}

  onModuleInit() {
    this.transporter = nodemailer.createTransport(
      {
        host: 'smtp.office365.com',
        secure: false,
        tls: {
          ciphers: 'SSLv3',
        },
        auth: {
          user: this.configService.get('MAIL_USER', { infer: true }),
          pass: this.configService.get('MAIL_PASS', { infer: true }),
        },
        logger: true,
        allowInternalNetworkInterfaces: false,
      })
   }

ERROR

user: this.configService.get('MAIL_USER', { infer: true }),
                                   ^
TypeError: Cannot read properties of undefined (reading 'get')

I was able to successfully retrieve environment variables with the get method in other methods that I had defined within the MailService class that I had created.