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 20b45882d..b664cfe7b 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
@@ -44,6 +44,7 @@
 package org.eclipse.jgit.transport;
 
 import static java.util.Collections.unmodifiableMap;
+import static java.util.Objects.requireNonNull;
 import static org.eclipse.jgit.lib.Constants.R_TAGS;
 import static org.eclipse.jgit.transport.GitProtocolConstants.CAPABILITY_REF_IN_WANT;
 import static org.eclipse.jgit.transport.GitProtocolConstants.CAPABILITY_SERVER_OPTION;
@@ -282,6 +283,8 @@ public class UploadPack {
 
 	private OutputStream msgOut = NullOutputStream.INSTANCE;
 
+	private ErrorWriter errOut = new PackProtocolErrorWriter();
+
 	/**
 	 * Refs eligible for advertising to the client, set using
 	 * {@link #setAdvertisedRefs}.
@@ -812,9 +815,9 @@ public class UploadPack {
 			// nothing.
 			throw err;
 		} catch (ServiceMayNotContinueException err) {
-			if (!err.isOutput() && err.getMessage() != null && pckOut != null) {
+			if (!err.isOutput() && err.getMessage() != null) {
 				try {
-					pckOut.writeString("ERR " + err.getMessage() + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
+					errOut.writeError(err.getMessage());
 				} catch (IOException e) {
 					err.addSuppressed(e);
 					throw err;
@@ -823,12 +826,12 @@ public class UploadPack {
 			}
 			throw err;
 		} catch (IOException | RuntimeException | Error err) {
-			if (pckOut != null) {
+			if (rawOut != null) {
 				String msg = err instanceof PackProtocolException
 						? err.getMessage()
 						: JGitText.get().internalServerError;
 				try {
-					pckOut.writeString("ERR " + msg + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
+					errOut.writeError(msg);
 				} catch (IOException e) {
 					err.addSuppressed(e);
 					throw err;
@@ -2115,6 +2118,8 @@ public class UploadPack {
 		boolean sideband = caps.contains(OPTION_SIDE_BAND)
 				|| caps.contains(OPTION_SIDE_BAND_64K);
 		if (sideband) {
+			errOut = new SideBandErrorWriter();
+
 			try {
 				sendPack(true, req, accumulator, allTags, unshallowCommits,
 						deepenNots, pckOut);
@@ -2124,7 +2129,7 @@ public class UploadPack {
 					message = JGitText.get().internalServerError;
 				}
 				try {
-					reportInternalServerErrorOverSideband(message);
+					errOut.writeError(message);
 				} catch (IOException e) {
 					err.addSuppressed(e);
 					throw err;
@@ -2132,8 +2137,7 @@ public class UploadPack {
 				throw new UploadPackInternalServerErrorException(err);
 			} catch (IOException | RuntimeException | Error err) {
 				try {
-					reportInternalServerErrorOverSideband(
-							JGitText.get().internalServerError);
+					errOut.writeError(JGitText.get().internalServerError);
 				} catch (IOException e) {
 					err.addSuppressed(e);
 					throw err;
@@ -2146,16 +2150,6 @@ public class UploadPack {
 		}
 	}
 
-	private void reportInternalServerErrorOverSideband(String message)
-			throws IOException {
-		@SuppressWarnings("resource" /* java 7 */)
-		SideBandOutputStream err = new SideBandOutputStream(
-				SideBandOutputStream.CH_ERROR, SideBandOutputStream.SMALL_BUF,
-				rawOut);
-		err.write(Constants.encode(message));
-		err.flush();
-	}
-
 	/**
 	 * Send the requested objects to the client.
 	 *
@@ -2411,4 +2405,28 @@ public class UploadPack {
 			}
 		}
 	}
+
+	private interface ErrorWriter {
+		void writeError(String message) throws IOException;
+	}
+
+	private class SideBandErrorWriter implements ErrorWriter {
+		@Override
+		public void writeError(String message) throws IOException {
+			@SuppressWarnings("resource" /* java 7 */)
+			SideBandOutputStream err = new SideBandOutputStream(
+					SideBandOutputStream.CH_ERROR,
+					SideBandOutputStream.SMALL_BUF, requireNonNull(rawOut));
+			err.write(Constants.encode(message));
+			err.flush();
+		}
+	}
+
+	private class PackProtocolErrorWriter implements ErrorWriter {
+		@Override
+		public void writeError(String message) throws IOException {
+			new PacketLineOut(requireNonNull(rawOut))
+					.writeString("ERR " + message + '\n'); //$NON-NLS-1$
+		}
+	}
 }