diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index c59ac9936a..5b0ba3ef20 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -351,14 +351,19 @@ ACK after 'done' if there is at least one common base and multi_ack or
 multi_ack_detailed is enabled. The server always sends NAK after 'done'
 if there is no common base found.
 
+Instead of 'ACK' or 'NAK', the server may send an error message (for
+example, if it does not recognize an object in a 'want' line received
+from the client).
+
 Then the server will start sending its packfile data.
 
 ----
-  server-response = *ack_multi ack / nak
+  server-response = *ack_multi ack / nak / error-line
   ack_multi       = PKT-LINE("ACK" SP obj-id ack_status)
   ack_status      = "continue" / "common" / "ready"
   ack             = PKT-LINE("ACK" SP obj-id)
   nak             = PKT-LINE("NAK")
+  error-line     =  PKT-LINE("ERR" SP explanation-text)
 ----
 
 A simple clone may look like this (with no 'have' lines):
diff --git a/fetch-pack.c b/fetch-pack.c
index f12bfcdbb1..986b1efebc 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -240,6 +240,8 @@ static enum ack_type get_ack(int fd, unsigned char *result_sha1)
 			return ACK;
 		}
 	}
+	if (skip_prefix(line, "ERR ", &arg))
+		die(_("remote error: %s"), arg);
 	die(_("git fetch-pack: expected ACK/NAK, got '%s'"), line);
 }