Keeps telling me I'm using a different type of IllegalArgumentException. Why? Please help

import java.util.Scanner;

public class TeacherAssistant {

  public static String validatedFieldName(String fieldName) {
    Scanner in = new Scanner(System.in);
    fieldName = "m"+in.nextLine();
    System.out.printf("Write a field NAME: %s \n", fieldName);
    char letter1 = fieldName.charAt(0);
    char letter2 = fieldName.charAt(1);
    boolean letterCheck1 = Character.isLowerCase(letter1);
    boolean letterCheck2 = Character.isUpperCase(letter2);
    boolean letterCheck3 = Character.isLetter(letter2);

    if (!letterCheck1){
    throw new IllegalArgumentException (letter1 +"You must start with an m");
    }

    if (!letterCheck2 && letterCheck3){
      throw new IllegalArgumentException (letter2 +"The second letter must be capital");
    }
    // These things should be verified:
    // 1.  Member fields must start with an 'm'
    // 2.  The second letter in the field name must be uppercased to ensure camel-casing
    // NOTE:  To check if something is not equal use the != symbol. eg: 3 != 4
    return fieldName;
  }

}