diff --git a/lib/std/Io/net.zig b/lib/std/Io/net.zig
index c704d8b9e924da4578e447f9d3e5be3ad9f42533..c32d7dfa5016d16ca6c79ba890c9372fec056c27 100644
--- a/lib/std/Io/net.zig
+++ b/lib/std/Io/net.zig
@@ -576,7 +576,7 @@ pub const Ip6Address = struct {
                         const name = text[text_i..];
                         if (name.len == 0) return .incomplete;
                         interface_name_text = name;
-                        text_i = @intCast(text.len);
+                        text_i = std.math.cast(u8, text.len) orelse return .{ .overflow = text.len };
                         continue :state .end;
                     },
                     else => return .{ .invalid_byte = text_i },
diff --git a/lib/std/Io/net/test.zig b/lib/std/Io/net/test.zig
index 1565904c574a6e392593dc7ad4b3c85a9ac1fd0a..308dc1a32670e207e72511d0128b634972050e6a 100644
--- a/lib/std/Io/net/test.zig
+++ b/lib/std/Io/net/test.zig
@@ -92,6 +92,11 @@ test "invalid but parseable IPv6 scope ids" {
     try testing.expectError(error.InterfaceNotFound, net.IpAddress.resolveIp6(io, "ff01::fb%123s45678901234", 0));
 }
 
+test "oversized IPv6 scope id" {
+    const long_scope: [256]u8 = @splat('a');
+    try testing.expectError(error.ParseFailed, net.IpAddress.resolveIp6(testing.io, "ff01::fb%" ++ &long_scope, 0));
+}
+
 test "parse and render IPv4 addresses" {
     try testIp4ParseAndRender("0.0.0.0");
     try testIp4ParseAndRender("255.255.255.255");