diff --git a/src/backend/replication/basebackup_copy.c b/src/backend/replication/basebackup_copy.c
index f42b368c03..8dfdff0644 100644
--- a/src/backend/replication/basebackup_copy.c
+++ b/src/backend/replication/basebackup_copy.c
@@ -152,18 +152,22 @@ bbsink_copystream_begin_backup(bbsink *sink)
 {
 	bbsink_copystream *mysink = (bbsink_copystream *) sink;
 	bbsink_state *state = sink->bbs_state;
+	char *buf;
 
 	/*
 	 * Initialize buffer. We ultimately want to send the archive and manifest
 	 * data by means of CopyData messages where the payload portion of each
-	 * message begins with a type byte, so we set up a buffer that begins with
-	 * a the type byte we're going to need, and then arrange things so that
-	 * the data we're given will be written just after that type byte. That
-	 * will allow us to ship the data with a single call to pq_putmessage and
-	 * without needing any extra copying.
+	 * message begins with a type byte. However, basebackup.c expects the
+	 * buffer to be aligned, so we can't just allocate one extra byte for the
+	 * type byte. Instead, allocate enough extra bytes that the portion of
+	 * the buffer we reveal to our callers can be aligned, while leaving room
+	 * to slip the type byte in just beforehand.  That will allow us to ship
+	 * the data with a single call to pq_putmessage and without needing any
+	 * extra copying.
 	 */
-	mysink->msgbuffer = palloc(mysink->base.bbs_buffer_length + 1);
-	mysink->base.bbs_buffer = mysink->msgbuffer + 1;
+	buf = palloc(mysink->base.bbs_buffer_length + MAXIMUM_ALIGNOF);
+	mysink->msgbuffer = buf + (MAXIMUM_ALIGNOF - 1);
+	mysink->base.bbs_buffer = buf + MAXIMUM_ALIGNOF;
 	mysink->msgbuffer[0] = 'd'; /* archive or manifest data */
 
 	/* Tell client the backup start location. */