diff --git a/src/string_immutable.zig b/src/string_immutable.zig
index 0d114024c8..38c01c4d80 100644
--- a/src/string_immutable.zig
+++ b/src/string_immutable.zig
@@ -2672,8 +2672,9 @@ pub fn escapeHTMLForLatin1Input(allocator: std.mem.Allocator, latin1: []const u8
 
                             buf = try std.ArrayList(u8).initCapacity(allocator, latin1.len + @as(usize, Scalar.lengths[c]));
                             const copy_len = @intFromPtr(ptr) - @intFromPtr(latin1.ptr);
-                            @memcpy(buf.items[0..copy_len], latin1[0..copy_len]);
+                            if (comptime Environment.allow_assert) assert(copy_len <= buf.capacity);
                             buf.items.len = copy_len;
+                            @memcpy(buf.items[0..copy_len], latin1[0..copy_len]);
                             any_needs_escape = true;
                             break :scan_and_allocate_lazily;
                         },