diff --git a/wp-includes/post.php b/wp-includes/post.php
index 582b94cee0..1cc76b9af9 100644
--- a/wp-includes/post.php
+++ b/wp-includes/post.php
@@ -4338,7 +4338,7 @@ function wp_mime_type_icon( $mime = 0 ) {
 					closedir($dh);
 				}
 			}
-			wp_cache_set( 'icon_files', $icon_files, 'default', 600 );
+			wp_cache_add( 'icon_files', $icon_files, 'default', 600 );
 		}
 
 		// Icon basename - extension = MIME wildcard