diff --git a/src/Umbraco.Web/PublishedCache/XmlPublishedCache/PublishedMediaCache.cs b/src/Umbraco.Web/PublishedCache/XmlPublishedCache/PublishedMediaCache.cs
index 6e3449e583..b4acd00889 100644
--- a/src/Umbraco.Web/PublishedCache/XmlPublishedCache/PublishedMediaCache.cs
+++ b/src/Umbraco.Web/PublishedCache/XmlPublishedCache/PublishedMediaCache.cs
@@ -539,6 +539,26 @@ namespace Umbraco.Web.PublishedCache.XmlPublishedCache
                     {
                         // use property type to ensure proper conversion
                         var propertyType = _contentType.GetPropertyType(i.Key);
+
+                        // note: this is where U4-4144 and -3665 were born
+                        //
+                        // because propertyType is null, the XmlPublishedProperty ctor will throw
+                        // it's null because i.Key is not a valid property alias for the type...
+                        // the alias is case insensitive (verified) so it means it really is not
+                        // a correct alias. 
+                        //
+                        // in every cases this is after a ConvertFromXPathNavigator, so it means
+                        // that we get some properties from the XML that are not valid properties.
+                        // no idea which property. could come from the cache in library, could come
+                        // from so many places really.
+
+                        // workaround: just ignore that property
+                        if (propertyType == null)
+                        {
+                            LogHelper.Warn<PublishedMediaCache>("Dropping property \"" + i.Key + "\" because it does not belong to the content type.");
+                            continue;
+                        }
+
                         property = new XmlPublishedProperty(propertyType, false, i.Value); // false :: never preview a media
                     }