why does it keep asking me if i put "clear:left;" on my code challenge?

a {
  text-decoration: none;
}

#wrapper {
  max-width: 940px;
  margin: 0 auto;
}

#logo {
  text-align: center;
  margin: 0;
}

h1, h2 {
  color: #fff;
}

nav a {
  color: #fff;
}

nav a:hover {
  color: #32673f;
}

h1 {
  font-family: Changa One, sans-serif;
  font-size: 1.75em;
  font-weight: normal;
}

img {
  max-width: 100%;
}

#gallery {
  margin: 0;
  padding: 0;
  list-style: none;
}

#gallery li {
  float: left;
  width: 45%;
  margin: 2.5%;
  background-color: #f5f5f5;
  color: #bdc3c7;
}

nav ul {
  list-style: none;
  margin: 0 10px;
  padding: 0;
}

nav li {
  display: inline-block;
}

nav a {
  font-weight: 800;
  padding: 15px 10px;
}

.profile-photo {
  display: block;
  margin: 0 auto 30px;
      max-width: 150px;
  border-radius: 100%;
}

.contact-info {
  list-style: none;
  padding: 0;
  margin: 0;
  font-size: 0.9em;
}

.contact-info a {
  display: block;
  min-height: 20px;
  background-repeat: no-repeat;
  background-size: 20px 20px;
  padding: 0 0 0 30px;
  margin: 0 0 10px;
}

@media screen and (min-width: 480px) {
#gallery li{
 width:28.3333%;   
}
#gallery li: nth-child(4n) {
  clear:left;
}
}