diff --git a/sdk/lib/_http/http_impl.dart b/sdk/lib/_http/http_impl.dart
index fa8a1226b1a..ff013ea463f 100644
--- a/sdk/lib/_http/http_impl.dart
+++ b/sdk/lib/_http/http_impl.dart
@@ -3744,7 +3744,7 @@ 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();
diff --git a/tests/standalone/io/http_proxy_test.dart b/tests/standalone/io/http_proxy_test.dart
index 3287a7012a4..038a627762a 100644
--- a/tests/standalone/io/http_proxy_test.dart
+++ b/tests/standalone/io/http_proxy_test.dart
@@ -296,21 +296,43 @@ Future<ProxyServer> setupProxyServer({ipV6 = false}) {
 testInvalidProxy() {
   HttpClient client = new HttpClient(context: clientContext);
 
+  // User without password.
+  client.findProxy = (Uri uri) => "PROXY user@localhost:80";
+  Future<HttpClientRequest?>.value(
+    client.getUrl(Uri.parse("http://www.google.com/test")),
+  ).catchError((error) {}, test: (e) => e is HttpException);
+
+  // User with empty password.
+  client.findProxy = (Uri uri) => "PROXY user:@localhost:80";
+  Future<HttpClientRequest?>.value(
+    client.getUrl(Uri.parse("http://www.google.com/test")),
+  ).catchError((error) {}, test: (e) => e is HttpException);
+
+  // User but no username.
+  client.findProxy = (Uri uri) => "PROXY :password@localhost:80";
+  Future<HttpClientRequest?>.value(
+    client.getUrl(Uri.parse("http://www.google.com/test")),
+  ).catchError((error) {}, test: (e) => e is HttpException);
+
+  // Empty proxy configuration.
   client.findProxy = (Uri uri) => "";
   Future<HttpClientRequest?>.value(
     client.getUrl(Uri.parse("http://www.google.com/test")),
   ).catchError((error) {}, test: (e) => e is HttpException);
 
+  // No 'PROXY' prefix.
   client.findProxy = (Uri uri) => "XXX";
   Future<HttpClientRequest?>.value(
     client.getUrl(Uri.parse("http://www.google.com/test")),
   ).catchError((error) {}, test: (e) => e is HttpException);
 
+  // No port.
   client.findProxy = (Uri uri) => "PROXY www.google.com";
   Future<HttpClientRequest?>.value(
     client.getUrl(Uri.parse("http://www.google.com/test")),
   ).catchError((error) {}, test: (e) => e is HttpException);
 
+  // Port string is non an integer.
   client.findProxy = (Uri uri) => "PROXY www.google.com:http";
   Future<HttpClientRequest?>.value(
     client.getUrl(Uri.parse("http://www.google.com/test")),