diff --git a/lib/std/io/Reader.zig b/lib/std/io/Reader.zig
index 096fe72afe0afdc1ee30a3ab66257c48c671cafa..952377f34bd0f7d6255d017ec93cfcc6dd2a1c65 100644
--- a/lib/std/io/Reader.zig
+++ b/lib/std/io/Reader.zig
@@ -131,8 +131,10 @@ pub const Limit = enum(usize) {
 };
 
 pub fn read(r: Reader, bw: *BufferedWriter, limit: Limit) RwError!usize {
+    const before = bw.count;
     const n = try r.vtable.read(r.context, bw, limit);
     assert(n <= @intFromEnum(limit));
+    assert(bw.count == before + n);
     return n;
 }