diff --git a/org.eclipse.jgit.http.test/tst/org/eclipse/jgit/http/test/SmartClientSmartServerTest.java b/org.eclipse.jgit.http.test/tst/org/eclipse/jgit/http/test/SmartClientSmartServerTest.java
index 82861ed..0f3d3c6 100644
--- a/org.eclipse.jgit.http.test/tst/org/eclipse/jgit/http/test/SmartClientSmartServerTest.java
+++ b/org.eclipse.jgit.http.test/tst/org/eclipse/jgit/http/test/SmartClientSmartServerTest.java
@@ -56,6 +56,7 @@
 import java.io.IOException;
 import java.io.PrintWriter;
 import java.net.URISyntaxException;
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -89,6 +90,8 @@
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.NullProgressMonitor;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectIdRef;
+import org.eclipse.jgit.lib.ObjectInserter;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.ReflogEntry;
 import org.eclipse.jgit.lib.ReflogReader;
@@ -487,6 +490,25 @@ public void testInitialClone_BrokenServer() throws Exception {
 	}
 
 	@Test
+	public void testInvalidWant() throws Exception {
+		@SuppressWarnings("resource")
+		ObjectId id = new ObjectInserter.Formatter().idFor(Constants.OBJ_BLOB,
+				"testInvalidWant".getBytes(StandardCharsets.UTF_8));
+
+		Repository dst = createBareRepository();
+		try (Transport t = Transport.open(dst, remoteURI);
+				FetchConnection c = t.openFetch()) {
+			Ref want = new ObjectIdRef.Unpeeled(Ref.Storage.NETWORK, id.name(),
+					id);
+			c.fetch(NullProgressMonitor.INSTANCE, Collections.singleton(want),
+					Collections.<ObjectId> emptySet());
+			fail("Server accepted want " + id.name());
+		} catch (TransportException err) {
+			assertEquals("want " + id.name() + " not valid", err.getMessage());
+		}
+	}
+
+	@Test
 	public void testPush_NotAuthorized() throws Exception {
 		final TestRepository src = createTestRepository();
 		final RevBlob Q_txt = src.blob("new text");
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
index 101057f..8b642bb 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
@@ -742,10 +742,6 @@ else if (requestValidator instanceof AnyRequestValidator)
 			if (!clientShallowCommits.isEmpty())
 				walk.assumeShallow(clientShallowCommits);
 			sendPack = negotiate();
-		} catch (PackProtocolException err) {
-			reportErrorDuringNegotiate(err.getMessage());
-			throw err;
-
 		} catch (ServiceMayNotContinueException err) {
 			if (!err.isOutput() && err.getMessage() != null) {
 				try {
@@ -756,15 +752,20 @@ else if (requestValidator instanceof AnyRequestValidator)
 				}
 			}
 			throw err;
-
-		} catch (IOException err) {
-			reportErrorDuringNegotiate(JGitText.get().internalServerError);
-			throw err;
-		} catch (RuntimeException err) {
-			reportErrorDuringNegotiate(JGitText.get().internalServerError);
-			throw err;
-		} catch (Error err) {
-			reportErrorDuringNegotiate(JGitText.get().internalServerError);
+		} catch (IOException | RuntimeException | Error err) {
+			boolean output = false;
+			try {
+				String msg = err instanceof PackProtocolException
+						? err.getMessage()
+						: JGitText.get().internalServerError;
+				pckOut.writeString("ERR " + msg + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
+				output = true;
+			} catch (Throwable err2) {
+				// Ignore this secondary failure, leave output false.
+			}
+			if (output) {
+				throw new UploadPackInternalServerErrorException(err);
+			}
 			throw err;
 		}
 
@@ -781,14 +782,6 @@ private static Set<ObjectId> refIdSet(Collection<Ref> refs) {
 		return ids;
 	}
 
-	private void reportErrorDuringNegotiate(String msg) {
-		try {
-			pckOut.writeString("ERR " + msg + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
-		} catch (Throwable err) {
-			// Ignore this secondary failure.
-		}
-	}
-
 	private void processShallow() throws IOException {
 		try (DepthWalk.RevWalk depthWalk = new DepthWalk.RevWalk(
 				walk.getObjectReader(), depth)) {
@@ -1186,9 +1179,7 @@ private void parseWants() throws IOException {
 			}
 			wantIds.clear();
 		} catch (MissingObjectException notFound) {
-			ObjectId id = notFound.getObjectId();
-			throw new PackProtocolException(MessageFormat.format(
-					JGitText.get().wantNotValid, id.name()), notFound);
+			throw new WantNotValidException(notFound.getObjectId(), notFound);
 		} finally {
 			q.release();
 		}
@@ -1213,8 +1204,7 @@ public void checkWants(UploadPack up, List<ObjectId> wants)
 			if (!up.isBiDirectionalPipe())
 				new ReachableCommitRequestValidator().checkWants(up, wants);
 			else if (!wants.isEmpty())
-				throw new PackProtocolException(MessageFormat.format(
-						JGitText.get().wantNotValid, wants.iterator().next().name()));
+				throw new WantNotValidException(wants.iterator().next());
 		}
 	}
 
@@ -1247,8 +1237,7 @@ else if (!wants.isEmpty()) {
 					refIdSet(up.getRepository().getRefDatabase().getRefs(ALL).values());
 				for (ObjectId obj : wants) {
 					if (!refIds.contains(obj))
-						throw new PackProtocolException(MessageFormat.format(
-								JGitText.get().wantNotValid, obj.name()));
+						throw new WantNotValidException(obj);
 				}
 			}
 		}
@@ -1294,14 +1283,11 @@ private static void checkNotAdvertisedWants(RevWalk walk,
 			RevObject obj;
 			while ((obj = q.next()) != null) {
 				if (!(obj instanceof RevCommit))
-					throw new PackProtocolException(MessageFormat.format(
-						JGitText.get().wantNotValid, obj.name()));
+					throw new WantNotValidException(obj);
 				walk.markStart((RevCommit) obj);
 			}
 		} catch (MissingObjectException notFound) {
-			ObjectId id = notFound.getObjectId();
-			throw new PackProtocolException(MessageFormat.format(
-					JGitText.get().wantNotValid, id.name()), notFound);
+			throw new WantNotValidException(notFound.getObjectId(), notFound);
 		} finally {
 			q.release();
 		}
@@ -1315,9 +1301,7 @@ private static void checkNotAdvertisedWants(RevWalk walk,
 
 		RevCommit bad = walk.next();
 		if (bad != null) {
-			throw new PackProtocolException(MessageFormat.format(
-					JGitText.get().wantNotValid,
-					bad.name()));
+			throw new WantNotValidException(bad);
 		}
 		walk.reset();
 	}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/WantNotValidException.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/WantNotValidException.java
new file mode 100644
index 0000000..f0f6434
--- /dev/null
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/WantNotValidException.java
@@ -0,0 +1,90 @@
+/*
+ * Copyright (C) 2016, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.transport;
+
+import java.text.MessageFormat;
+
+import org.eclipse.jgit.errors.PackProtocolException;
+import org.eclipse.jgit.internal.JGitText;
+import org.eclipse.jgit.lib.AnyObjectId;
+
+/**
+ * Indicates client requested an object the server does not want to serve.
+ * <p>
+ * Typically visible only inside of the server implementation; clients are
+ * usually looking at the text message from the server in a generic
+ * {@link PackProtocolException}.
+ *
+ * @since 4.3
+ */
+public class WantNotValidException extends PackProtocolException {
+	private static final long serialVersionUID = 1L;
+
+	/**
+	 * Construct a {@code "want $id not valid"} exception.
+	 *
+	 * @param id
+	 *            invalid object identifier received from the client.
+	 */
+	public WantNotValidException(AnyObjectId id) {
+		super(msg(id));
+	}
+
+	/**
+	 * Construct a {@code "want $id not valid"} exception.
+	 *
+	 * @param id
+	 *            invalid object identifier received from the client.
+	 * @param cause
+	 *            root cause of the object being invalid, such as an IOException
+	 *            from the storage system.
+	 */
+	public WantNotValidException(AnyObjectId id, Throwable cause) {
+		super(msg(id), cause);
+	}
+
+	private static String msg(AnyObjectId id) {
+		return MessageFormat.format(JGitText.get().wantNotValid, id.name());
+	}
+}