diff --git a/src/apprt/surface.zig b/src/apprt/surface.zig
index c7ebc6ac8..58e3cea9b 100644
--- a/src/apprt/surface.zig
+++ b/src/apprt/surface.zig
@@ -9,7 +9,7 @@ const Config = @import("../config.zig").Config;
 pub const Message = union(enum) {
     /// Represents a write request. Magic number comes from the max size
     /// we want this union to be.
-    pub const WriteReq = termio.MessageData(u8, 256);
+    pub const WriteReq = termio.MessageData(u8, 255);
 
     /// Set the title of the surface.
     /// TODO: we should change this to a "WriteReq" style structure in
diff --git a/src/termio/message.zig b/src/termio/message.zig
index c7fe19976..d0212950e 100644
--- a/src/termio/message.zig
+++ b/src/termio/message.zig
@@ -84,6 +84,8 @@ pub const Message = union(enum) {
 /// are a stable pointer, or require deallocation. This is helpful for thread
 /// messaging utilities.
 pub fn MessageData(comptime Elem: type, comptime small_size: comptime_int) type {
+    assert(small_size <= std.math.maxInt(u8));
+
     return union(enum) {
         pub const Self = @This();