Why does this give compiler error? Validation Code Challenge

public class TeacherAssistant {

  public static String validatedFieldName(String fieldName) {
    // These things should be verified:
    // 1.  Member fields must start with an 'm'public static String validatedFieldName(String fieldName) {

    if ( fieldName.charAt(0) != 'm'){
      throw new IllegalArgumentException("error");
    } 

    // 2.  The second letter in the field name must be uppercased to ensure camel-casing
     if ( !Character.isUpperCase(fieldName.charAt(1)){
        throw new IllegalArgumentException("error");
    }
    // NOTE:  To check if something is not equal use the != symbol. eg: 3 != 4
    return fieldName;
  }

}