Expected "m_first_name" to fail but it passed

public class TeacherAssistant {

  public static String validatedFieldName(String fieldName) {
    // 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

    if (fieldName.charAt(0) != 'm') {
throw new IllegalArgumentException("m letter");
    }
    char secLetter = fieldName.charAt(1);

    if (Character.isLowerCase(secLetter)) {
      throw new IllegalArgumentException("lowercase letter");
    }

    return fieldName;
  }

}