diff --git a/Observatory/ScanEvent.cs b/Observatory/ScanEvent.cs
index 7084784..3d9d500 100644
--- a/Observatory/ScanEvent.cs
+++ b/Observatory/ScanEvent.cs
@@ -295,7 +295,7 @@ public partial class Ring
         public string RingClass { get; set; }
 
         [JsonProperty("MassMT", NullValueHandling = NullValueHandling.Ignore)]
-        public long? MassMT { get; set; }
+        public ulong? MassMT { get; set; }
 
         [JsonProperty("InnerRad", NullValueHandling = NullValueHandling.Ignore)]
         public long? InnerRad { get; set; }