diff --git a/src/struct.zig b/src/struct.zig
index d1b9f29..2e9e1c6 100644
--- a/src/struct.zig
+++ b/src/struct.zig
@@ -144,7 +144,7 @@ pub fn packStruct(writer: *std.Io.Writer, comptime T: type, value_or_maybe_null:
     if (has_custom_write_fn) {
         return try value.msgpackWrite(packer(writer));
     } else {
-        const format = comptime if (std.meta.hasFn(Type, "msgpackFormat")) T.msgpackFormat() else default_struct_format;
+        const format = comptime if (std.meta.hasFn(Type, "msgpackFormat")) Type.msgpackFormat() else default_struct_format;
         switch (format) {
             .as_map => |opts| {
                 return packStructAsMap(writer, Type, value, opts);
@@ -160,7 +160,7 @@ pub fn unpackStructFromMapBody(reader: *std.Io.Reader, allocator: std.mem.Alloca
     const Type = NonOptional(T);
     const type_info = @typeInfo(Type);
     const fields = type_info.@"struct".fields;
-    const FieldEnum = std.meta.FieldEnum(T);
+    const FieldEnum = std.meta.FieldEnum(Type);
 
     var fields_seen = std.bit_set.StaticBitSet(fields.len).initEmpty();
 
@@ -208,10 +208,10 @@ pub fn unpackStructFromMapBody(reader: *std.Io.Reader, allocator: std.mem.Alloca
                 }
             },
             .custom => {
-                const KeyType = comptime @typeInfo(@TypeOf(T.msgpackFieldKey)).@"fn".return_type.?;
+                const KeyType = comptime @typeInfo(@TypeOf(Type.msgpackFieldKey)).@"fn".return_type.?;
                 const key = try unpackAny(reader, allocator, KeyType);
                 inline for (fields, 0..) |field, i| {
-                    if (T.msgpackFieldKey(@field(FieldEnum, field.name)) == key) {
+                    if (Type.msgpackFieldKey(@field(FieldEnum, field.name)) == key) {
                         fields_seen.set(i);
                         @field(result, field.name) = try unpackAny(reader, allocator, field.type);
                         break;
@@ -280,9 +280,9 @@ pub fn unpackStruct(reader: *std.Io.Reader, allocator: std.mem.Allocator, compti
 
     const has_custom_read_fn = std.meta.hasFn(Type, "msgpackRead");
     if (has_custom_read_fn) {
-        return try T.msgpackRead(unpacker(reader, allocator));
+        return try Type.msgpackRead(unpacker(reader, allocator));
     } else {
-        const format = comptime if (std.meta.hasFn(Type, "msgpackFormat")) T.msgpackFormat() else default_struct_format;
+        const format = comptime if (std.meta.hasFn(Type, "msgpackFormat")) Type.msgpackFormat() else default_struct_format;
         switch (format) {
             .as_map => |opts| {
                 return try unpackStructAsMap(reader, allocator, T, opts);
@@ -318,6 +318,68 @@ test "writeStruct: map_by_index" {
     }, writer.buffered());
 }
 
+test "writeStruct: optional custom format" {
+    const Msg = struct {
+        a: u32,
+
+        pub fn msgpackFormat() StructFormat {
+            return .{ .as_map = .{ .key = .field_index } };
+        }
+    };
+
+    var buffer: [100]u8 = undefined;
+    var writer = std.Io.Writer.fixed(&buffer);
+    try packStruct(&writer, ?Msg, Msg{ .a = 1 });
+
+    try std.testing.expectEqualSlices(u8, &.{
+        0x81,
+        0x00,
+        0x01,
+    }, writer.buffered());
+}
+
+test "readStruct: optional custom field key" {
+    const Msg = struct {
+        a: u32,
+
+        pub fn msgpackFormat() StructFormat {
+            return .{ .as_map = .{ .key = .custom } };
+        }
+
+        pub fn msgpackFieldKey(field: std.meta.FieldEnum(@This())) u8 {
+            return switch (field) {
+                .a => 1,
+            };
+        }
+    };
+
+    const packed_msg = [_]u8{
+        0x81,
+        0x01,
+        0x2a,
+    };
+    var reader = std.Io.Reader.fixed(&packed_msg);
+    const decoded = try unpackStruct(&reader, std.testing.allocator, ?Msg);
+
+    try std.testing.expectEqual(@as(?Msg, Msg{ .a = 42 }), decoded);
+}
+
+test "readStruct: optional custom reader" {
+    const Msg = struct {
+        a: u32,
+
+        pub fn msgpackRead(unpacker_value: anytype) !@This() {
+            return .{ .a = try unpacker_value.readInt(u32) };
+        }
+    };
+
+    const packed_msg = [_]u8{0x2a};
+    var reader = std.Io.Reader.fixed(&packed_msg);
+    const decoded = try unpackStruct(&reader, std.testing.allocator, ?Msg);
+
+    try std.testing.expectEqual(@as(?Msg, Msg{ .a = 42 }), decoded);
+}
+
 test "writeStruct: map by field_name" {
     const Msg = struct {
         a: u32,
diff --git a/src/union.zig b/src/union.zig
index 1758529..d473614 100644
--- a/src/union.zig
+++ b/src/union.zig
@@ -139,7 +139,7 @@ pub fn packUnion(writer: *std.Io.Writer, comptime T: type, value_or_maybe_null:
         @compileError("Expected union type");
     }
 
-    const format = if (std.meta.hasFn(Type, "msgpackFormat")) T.msgpackFormat() else default_union_format;
+    const format = if (std.meta.hasFn(Type, "msgpackFormat")) Type.msgpackFormat() else default_union_format;
     switch (format) {
         .as_map => |opts| {
             return packUnionAsMap(writer, Type, value, opts);
@@ -281,7 +281,7 @@ pub fn unpackUnionAsTagged(reader: *std.Io.Reader, allocator: std.mem.Allocator,
 pub fn unpackUnion(reader: *std.Io.Reader, allocator: std.mem.Allocator, comptime T: type) !T {
     const Type = NonOptional(T);
 
-    const format = if (std.meta.hasFn(Type, "msgpackFormat")) T.msgpackFormat() else default_union_format;
+    const format = if (std.meta.hasFn(Type, "msgpackFormat")) Type.msgpackFormat() else default_union_format;
     switch (format) {
         .as_map => |opts| {
             return try unpackUnionAsMap(reader, allocator, T, opts);
@@ -329,6 +329,48 @@ test "writeUnion: int field" {
     try std.testing.expectEqualSlices(u8, &msg1_packed, writer.buffered());
 }
 
+test "writeUnion: optional custom format" {
+    const Msg = union(enum) {
+        a: u32,
+        b: u64,
+
+        pub fn msgpackFormat() UnionFormat {
+            return .{ .as_map = .{ .key = .field_index } };
+        }
+    };
+
+    var buffer: [100]u8 = undefined;
+    var writer = std.Io.Writer.fixed(&buffer);
+    try packUnion(&writer, ?Msg, Msg{ .a = 1 });
+
+    try std.testing.expectEqualSlices(u8, &.{
+        0x81,
+        0x00,
+        0x01,
+    }, writer.buffered());
+}
+
+test "readUnion: optional custom format" {
+    const Msg = union(enum) {
+        a: u32,
+        b: u64,
+
+        pub fn msgpackFormat() UnionFormat {
+            return .{ .as_map = .{ .key = .field_index } };
+        }
+    };
+
+    const packed_msg = [_]u8{
+        0x81,
+        0x00,
+        0x2a,
+    };
+    var reader = std.Io.Reader.fixed(&packed_msg);
+    const decoded = try unpackUnion(&reader, std.testing.allocator, ?Msg);
+
+    try std.testing.expectEqualDeep(@as(?Msg, Msg{ .a = 42 }), decoded);
+}
+
 test "writeUnion: void field" {
     var buffer: [100]u8 = undefined;
     var writer = std.Io.Writer.fixed(&buffer);