diff --git a/fixed_buffer_stream.zig b/fixed_buffer_stream.zig
index 9af8fbe6d2b3a40968ef435e76f375bac670b8fb..82b1f100de7a4c282ffb4d187c57bb3796b86d22 100644
--- a/fixed_buffer_stream.zig
+++ b/fixed_buffer_stream.zig
@@ -98,7 +98,7 @@ pub fn FixedBufferStream(comptime Buffer: type) type {
 
         pub fn anyWritable(self: *Self) nio.AnyWritable {
             const S = struct {
-                fn write(s: *allowzero anyopaque, buffer: []u8) anyerror!usize {
+                fn write(s: *allowzero anyopaque, buffer: []const u8) anyerror!usize {
                     const fbs: *Self = @ptrCast(@alignCast(s));
                     return fbs.write(buffer);
                 }