ResetPassword fails despite resetting the password
<membership defaultProvider="AspNetSqlMembershipProvider">
  <providers>
    <clear/>
      <add name="AspNetSqlMembershipProvider"
           type="System.Web.Security.SqlMembershipProvider"
           passwordFormat="Hashed"
           connectionStringName="SqlConnectionString"
           enablePasswordRetrieval="false"
           enablePasswordReset="true"
           requiresQuestionAndAnswer="false"
           requiresUniqueEmail="true"
           maxInvalidPasswordAttempts="5"
           minRequiredPasswordLength="3"
           minRequiredNonalphanumericCharacters="0"
           passwordAttemptWindow="10"
           applicationName="/"/>
  </providers>
</membership>

public static string ResetPassword(string username)
{
    MembershipUser user = Membership.GetUser(username);
    if (user != null)
    {
        string resetPassword = user.ResetPassword(); //This line throws the exception
        /* SNIP - Emailing code */
        return "Your password has been reset and emailed to you.";
    }
    else
    {
        return "Password could not be reset. Please check the email address you entered was correct.";
    };
}