Misread the error, my bad

src/Discord.Net.Core/Entities/Guilds/IUserGuild.cs

@@ -9,7 +9,7 @@ namespace Discord
         /// <summary>
         ///     Gets the icon URL associated with this guild, or <c>null</c> if one is not set.
         /// </summary>
-        string IconUrl { get; }
+        string GetIconUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128);
         /// <summary>
         ///     Returns <c>true</c> if the current user owns this guild.
         /// </summary>

src/Discord.Net.Rest/Entities/Guilds/RestUserGuild.cs

@@ -21,7 +21,7 @@ namespace Discord.Rest
         public DateTimeOffset CreatedAt => SnowflakeUtils.FromSnowflake(Id);
         /// <inheritdoc />
         public string GetIconUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
-            => CDN.GetGuildIconUrl(Id, IconId, size, format);
+            => CDN.GetGuildIconUrl(Id, _iconId, size, format);
 
         internal RestUserGuild(BaseDiscordClient discord, ulong id)
             : base(discord, id)