diff --git a/src/DuetAPI/Commands/Code/Code.cs b/src/DuetAPI/Commands/Code/Code.cs
index f25fe8fa9..8e0751587 100644
--- a/src/DuetAPI/Commands/Code/Code.cs
+++ b/src/DuetAPI/Commands/Code/Code.cs
@@ -95,7 +95,7 @@ public Code(string code)
         /// <summary>
         /// Minor code number (e.g. 3 in G54.3)
         /// </summary>
-        public sbyte? MinorNumber { get; set; }
+        public int? MinorNumber { get; set; }
 
         /// <summary>
         /// Flags of this code
diff --git a/src/DuetAPI/Commands/Code/Parser.cs b/src/DuetAPI/Commands/Code/Parser.cs
index 3b836edf1..8287d0cc0 100644
--- a/src/DuetAPI/Commands/Code/Parser.cs
+++ b/src/DuetAPI/Commands/Code/Parser.cs
@@ -457,7 +457,7 @@ public static bool Parse(TextReader reader, Code result)
                                 {
                                     throw new CodeParserException($"Failed to parse major {char.ToUpperInvariant((char)result.Type)}-code number ({args[0]})", result);
                                 }
-                                if (sbyte.TryParse(args[1], out sbyte minorNumber) && minorNumber >= 0)
+                                if (int.TryParse(args[1], out int minorNumber) && minorNumber >= 0)
                                 {
                                     result.MinorNumber = minorNumber;
                                 }
diff --git a/src/DuetAPI/Commands/Code/ParserAsync.cs b/src/DuetAPI/Commands/Code/ParserAsync.cs
index 7c953f6c1..c26879bfc 100644
--- a/src/DuetAPI/Commands/Code/ParserAsync.cs
+++ b/src/DuetAPI/Commands/Code/ParserAsync.cs
@@ -597,7 +597,7 @@ void clearValue()
                                 {
                                     throw new CodeParserException($"Failed to parse major {char.ToUpperInvariant((char)result.Type)}-code number ({args[0]})", result);
                                 }
-                                if (sbyte.TryParse(args[1], out sbyte minorNumber) && minorNumber >= 0)
+                                if (int.TryParse(args[1], out int minorNumber) && minorNumber >= 0)
                                 {
                                     result.MinorNumber = minorNumber;
                                 }
diff --git a/src/DuetControlServer/SPI/Serialization/Writer.cs b/src/DuetControlServer/SPI/Serialization/Writer.cs
index 671ce3aaa..97a5c2efc 100644
--- a/src/DuetControlServer/SPI/Serialization/Writer.cs
+++ b/src/DuetControlServer/SPI/Serialization/Writer.cs
@@ -72,8 +72,8 @@ public static int WriteCode(Span<byte> to, Code code)
                 Channel = code.Channel,
                 FilePosition = (uint)(code.FilePosition ?? 0xFFFFFFFF),
                 Letter = (byte)code.Type,
-                MajorCode = (code.Type == CodeType.Comment) ? 0 : (code.MajorNumber ?? -1),
-                MinorCode = code.MinorNumber ?? -1,
+                MajorCode = (code.Type == CodeType.Comment) ? 0 : (code.MajorNumber ?? 0),
+                MinorCode = code.MinorNumber ?? 0,
                 NumParameters = (byte)((code.Type == CodeType.Comment) ? 1 : code.Parameters.Count)
             };