Java - Test whether a String is valid or not

This is what the instructions tells me:

"So I was wondering if you could help me with some of these code challenges. As you might've seen when a member field name doesn't match the style we've talked about, I send an error about it. Let's write some code to validate the style of the field name.

I've added the method validatedFieldName it will return the validated field name. If the value passed in doesn't meet the requirements, throw an IllegalArgumentException. I'll keep you posted on the results every time you press check work."

The error message tells me: "Bummer! Expected "m_first_name" to fail but passed"

I hope you can help me :)

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' || fieldName.charAt(1) != Character.toUpperCase(fieldName.charAt(1) ) )
    {
      throw new IllegalArgumentException("Invalid field name!");
    }

    return fieldName;
  }

}