diff --git a/src/Umbraco.Core/PropertyEditors/ValueConverters/MustBeStringValueConverter.cs b/src/Umbraco.Core/PropertyEditors/ValueConverters/MustBeStringValueConverter.cs
new file mode 100644
index 0000000000..45563d11e8
--- /dev/null
+++ b/src/Umbraco.Core/PropertyEditors/ValueConverters/MustBeStringValueConverter.cs
@@ -0,0 +1,38 @@
+using System.Linq;
+using Umbraco.Core.Models.PublishedContent;
+
+namespace Umbraco.Core.PropertyEditors.ValueConverters
+{
+    /// <summary>
+    /// Ensures that no matter what is selected in (editor), the value results in a string.
+    /// </summary>
+    /// <remarks>
+    /// <para>For more details see issues http://issues.umbraco.org/issue/U4-3776 (MNTP)
+    /// and http://issues.umbraco.org/issue/U4-4160 (media picker).</para>
+    /// <para>The cache level is set to .Content because the string is supposed to depend
+    /// on the source value only, and not on any other content. It is NOT appropriate
+    /// to use that converter for values whose .ToString() would depend on other content.</para>
+    /// </remarks>
+    [DefaultPropertyValueConverter]
+    [PropertyValueType(typeof(string))]
+    [PropertyValueCache(PropertyCacheValue.All, PropertyCacheLevel.Content)]
+    public class MustBeStringValueConverter : PropertyValueConverterBase
+    {
+        private static readonly string[] Aliases =
+        {
+            Constants.PropertyEditors.MultiNodeTreePickerAlias,
+            Constants.PropertyEditors.MultipleMediaPickerAlias
+        };
+
+        public override bool IsConverter(PublishedPropertyType propertyType)
+        {
+            return Aliases.Contains(propertyType.PropertyEditorAlias);
+        }
+
+        public override object ConvertDataToSource(PublishedPropertyType propertyType, object source, bool preview)
+        {
+            if (source == null) return null;
+            return source.ToString();
+        }
+    }
+}
diff --git a/src/Umbraco.Core/Umbraco.Core.csproj b/src/Umbraco.Core/Umbraco.Core.csproj
index 18cd086cbe..f29a9f5b29 100644
--- a/src/Umbraco.Core/Umbraco.Core.csproj
+++ b/src/Umbraco.Core/Umbraco.Core.csproj
@@ -368,6 +368,7 @@
     <Compile Include="PropertyEditors\ValueConverters\JsonValueConverter.cs" />
     <Compile Include="PropertyEditors\ValueConverters\MultipleTextStringValueConverter.cs" />
     <Compile Include="PropertyEditors\ValueConverters\MarkdownEditorValueConverter.cs" />
+    <Compile Include="PropertyEditors\ValueConverters\MustBeStringValueConverter.cs" />
     <Compile Include="PropertyEditors\ValueConverters\TextStringValueConverter.cs" />
     <Compile Include="PropertyEditors\ValueConverters\TinyMceValueConverter.cs" />
     <Compile Include="PropertyEditors\IPropertyValueConverter.cs" />
diff --git a/src/Umbraco.Web/PropertyEditors/ValueConverters/MntpStringValueConverter.cs b/src/Umbraco.Web/PropertyEditors/ValueConverters/MntpStringValueConverter.cs
deleted file mode 100644
index 14cc3a9b57..0000000000
--- a/src/Umbraco.Web/PropertyEditors/ValueConverters/MntpStringValueConverter.cs
+++ /dev/null
@@ -1,29 +0,0 @@
-using Umbraco.Core;
-using Umbraco.Core.Models.PublishedContent;
-using Umbraco.Core.PropertyEditors;
-
-namespace Umbraco.Web.PropertyEditors.ValueConverters
-{
-    /// <summary>
-    /// Ensures that no matter what is selected in MNTP that the value results in a string
-    /// </summary>
-    /// <remarks>
-    /// See here for full details:http://issues.umbraco.org/issue/U4-3776
-    /// </remarks>
-    [DefaultPropertyValueConverter]
-    [PropertyValueType(typeof (string))]
-    [PropertyValueCache(PropertyCacheValue.All, PropertyCacheLevel.Request)]
-    public class MntpStringValueConverter : PropertyValueConverterBase
-    {        
-        public override bool IsConverter(PublishedPropertyType propertyType)
-        {
-            return propertyType.PropertyEditorAlias == Constants.PropertyEditors.MultiNodeTreePickerAlias;
-        }
-
-        public override object ConvertDataToSource(PublishedPropertyType propertyType, object source, bool preview)
-        {
-            if (source == null) return null;
-            return source.ToString();
-        }
-    }
-}
\ No newline at end of file
diff --git a/src/Umbraco.Web/Umbraco.Web.csproj b/src/Umbraco.Web/Umbraco.Web.csproj
index ce74fb02a8..a2b131709d 100644
--- a/src/Umbraco.Web/Umbraco.Web.csproj
+++ b/src/Umbraco.Web/Umbraco.Web.csproj
@@ -397,7 +397,6 @@
     <Compile Include="PropertyEditors\TagsPropertyEditor.cs" />
     <Compile Include="PropertyEditors\UploadFileTypeValidator.cs" />
     <Compile Include="PropertyEditors\UserPickerPropertyEditor.cs" />
-    <Compile Include="PropertyEditors\ValueConverters\MntpStringValueConverter.cs" />
     <Compile Include="PropertyEditors\ValueConverters\RelatedLinksEditorValueConvertor.cs" />
     <Compile Include="PropertyEditors\ValueListPreValueEditor.cs" />
     <Compile Include="PropertyEditors\DropDownPropertyEditor.cs" />