diff --git a/SabreTools.Library/DatFiles/Listxml.cs b/SabreTools.Library/DatFiles/Listxml.cs
index b2cb6a31..fe3f2253 100644
--- a/SabreTools.Library/DatFiles/Listxml.cs
+++ b/SabreTools.Library/DatFiles/Listxml.cs
@@ -276,7 +276,7 @@ namespace SabreTools.Library.DatFiles
                             DeviceType = reader.GetAttribute("type").AsDeviceType(),
                             Tag = reader.GetAttribute("tag"),
                             FixedImage = reader.GetAttribute("fixed_image"),
-                            Mandatory = reader.GetAttribute("mandatory"),
+                            Mandatory = Sanitizer.CleanLong(reader.GetAttribute("mandatory")),
                             Interface = reader.GetAttribute("interface"),
 
                             Source = new Source
@@ -1421,7 +1421,7 @@ namespace SabreTools.Library.DatFiles
                         xtw.WriteOptionalAttributeString("type", device.DeviceType.FromDeviceType());
                         xtw.WriteOptionalAttributeString("tag", device.Tag);
                         xtw.WriteOptionalAttributeString("fixed_image", device.FixedImage);
-                        xtw.WriteOptionalAttributeString("mandatory", device.Mandatory);
+                        xtw.WriteOptionalAttributeString("mandatory", device.Mandatory?.ToString());
                         xtw.WriteOptionalAttributeString("interface", device.Interface);
                         if (device.Instances != null)
                         {
diff --git a/SabreTools.Library/DatFiles/SabreDat.cs b/SabreTools.Library/DatFiles/SabreDat.cs
index c050ad13..e06e5c5b 100644
--- a/SabreTools.Library/DatFiles/SabreDat.cs
+++ b/SabreTools.Library/DatFiles/SabreDat.cs
@@ -1324,7 +1324,7 @@ namespace SabreTools.Library.DatFiles
                         xtw.WriteOptionalAttributeString("type", device.DeviceType.FromDeviceType());
                         xtw.WriteOptionalAttributeString("tag", device.Tag);
                         xtw.WriteOptionalAttributeString("fixed_image", device.FixedImage);
-                        xtw.WriteOptionalAttributeString("mandatory", device.Mandatory);
+                        xtw.WriteOptionalAttributeString("mandatory", device.Mandatory?.ToString());
                         xtw.WriteOptionalAttributeString("interface", device.Interface);
                         if (device.Instances != null)
                         {
diff --git a/SabreTools.Library/DatItems/Device.cs b/SabreTools.Library/DatItems/Device.cs
index 0850a991..d214fbf9 100644
--- a/SabreTools.Library/DatItems/Device.cs
+++ b/SabreTools.Library/DatItems/Device.cs
@@ -38,7 +38,7 @@ namespace SabreTools.Library.DatItems
         /// </summary>
         /// <remarks>Only value used seems to be 1. Used like bool, but actually int</remarks>
         [JsonProperty("mandatory", DefaultValueHandling = DefaultValueHandling.Ignore)]
-        public string Mandatory { get; set; } // TODO: long?
+        public long? Mandatory { get; set; }
 
         /// <summary>
         /// Device interface
@@ -82,7 +82,7 @@ namespace SabreTools.Library.DatItems
                 FixedImage = mappings[Field.DatItem_FixedImage];
 
             if (mappings.Keys.Contains(Field.DatItem_Mandatory))
-                Mandatory = mappings[Field.DatItem_Mandatory];
+                Mandatory = Sanitizer.CleanLong(mappings[Field.DatItem_Mandatory]);
 
             if (mappings.Keys.Contains(Field.DatItem_Interface))
                 Interface = mappings[Field.DatItem_Interface];
@@ -218,9 +218,11 @@ namespace SabreTools.Library.DatItems
                 return false;
 
             // Filter on mandatory
-            if (filter.DatItem_Mandatory.MatchesPositiveSet(Mandatory) == false)
+            if (filter.DatItem_Mandatory.MatchesNeutral(null, Mandatory) == false)
                 return false;
-            if (filter.DatItem_Mandatory.MatchesNegativeSet(Mandatory) == true)
+            else if (filter.DatItem_Mandatory.MatchesPositive(null, Mandatory) == false)
+                return false;
+            else if (filter.DatItem_Mandatory.MatchesNegative(null, Mandatory) == false)
                 return false;
 
             // Filter on interface
diff --git a/SabreTools.Library/Filtering/Filter.cs b/SabreTools.Library/Filtering/Filter.cs
index 13db577a..b8ae8df3 100644
--- a/SabreTools.Library/Filtering/Filter.cs
+++ b/SabreTools.Library/Filtering/Filter.cs
@@ -191,7 +191,7 @@ namespace SabreTools.Library.Filtering
         // Device
         public FilterItem<DeviceType> DatItem_DeviceType { get; private set; } = new FilterItem<DeviceType>() { Positive = DeviceType.NULL, Negative = DeviceType.NULL };
         public FilterItem<string> DatItem_FixedImage { get; private set; } = new FilterItem<string>();
-        public FilterItem<string> DatItem_Mandatory { get; private set; } = new FilterItem<string>();
+        public FilterItem<long?> DatItem_Mandatory { get; private set; } = new FilterItem<long?>() { Positive = null, Negative = null, Neutral = null };
         public FilterItem<string> DatItem_Interface { get; private set; } = new FilterItem<string>();
 
         // Display
@@ -826,7 +826,7 @@ namespace SabreTools.Library.Filtering
                     break;
 
                 case Field.DatItem_Mandatory:
-                    SetStringFilter(DatItem_Mandatory, value, negate);
+                    SetLongFilter(DatItem_Mandatory, value, negate);
                     break;
 
                 case Field.DatItem_Interface: