diff --git a/sdk/lib/_http/http_impl.dart b/sdk/lib/_http/http_impl.dart
index c3405e5a4020..45d11b122fe1 100644
--- a/sdk/lib/_http/http_impl.dart
+++ b/sdk/lib/_http/http_impl.dart
@@ -3746,7 +3746,7 @@ class _ProxyConfiguration {
             String userinfo = proxy.substring(0, at).trim();
             proxy = proxy.substring(at + 1).trim();
             int colon = userinfo.indexOf(":");
-            if (colon == -1 || colon == 0 || colon == proxy.length - 1) {
+            if (colon == -1 || colon == 0 || colon == userinfo.length - 1) {
               throw HttpException("Invalid proxy configuration $configuration");
             }
             username = userinfo.substring(0, colon).trim();