--- a/src/jdk.incubator.httpclient/share/classes/jdk/incubator/http/internal/websocket/BuilderImpl.java	Sun Nov 05 17:32:13 2017 +0000
+++ b/src/jdk.incubator.httpclient/share/classes/jdk/incubator/http/internal/websocket/BuilderImpl.java	Sun Nov 05 21:19:55 2017 +0000
@@ -38,6 +38,7 @@
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
 
+import static java.lang.String.format;
 import static java.util.Objects.requireNonNull;
 import static jdk.incubator.http.internal.common.Pair.pair;
 
@@ -52,10 +53,28 @@
 
     public BuilderImpl(HttpClient client, URI uri, Listener listener) {
         this.client = requireNonNull(client, "client");
-        this.uri = requireNonNull(uri, "uri");
+        this.uri = checkURI(requireNonNull(uri, "uri"));
         this.listener = requireNonNull(listener, "listener");
     }
 
+    private static IllegalArgumentException newIAE(String message, Object... args) {
+        return new IllegalArgumentException(format(message, args));
+    }
+
+    private static URI checkURI(URI uri) {
+        String scheme = uri.getScheme();
+        if (scheme == null)
+            throw newIAE("URI with undefined scheme");
+        scheme = scheme.toLowerCase();
+        if (!(scheme.equals("ws") || scheme.equals("wss")))
+            throw newIAE("invalid URI scheme %s", scheme);
+        if (uri.getHost() == null)
+            throw newIAE("URI must contain a host: %s", uri);
+        if (uri.getFragment() != null)
+            throw newIAE("URI must not contain a fragment: %s", uri);
+        return uri;
+    }
+
     @Override
     public Builder header(String name, String value) {
         requireNonNull(name, "name");