diff --git a/Documentation/git-update-ref.txt b/Documentation/git-update-ref.txt
index 0a0a5512b3..c8f5ae5cb3 100644
--- a/Documentation/git-update-ref.txt
+++ b/Documentation/git-update-ref.txt
@@ -68,7 +68,12 @@ performs all modifications together.  Specify commands of the form:
 	option SP <opt> LF
 
 Quote fields containing whitespace as if they were strings in C source
-code.  Alternatively, use `-z` to specify commands without quoting:
+code; i.e., surrounded by double-quotes and with backslash escapes.
+Use 40 "0" characters or the empty string to specify a zero value.  To
+specify a missing value, omit the value and its preceding SP entirely.
+
+Alternatively, use `-z` to specify in NUL-terminated format, without
+quoting:
 
 	update SP <ref> NUL <newvalue> NUL [<oldvalue>] NUL
 	create SP <ref> NUL <newvalue> NUL
@@ -76,8 +81,12 @@ code.  Alternatively, use `-z` to specify commands without quoting:
 	verify SP <ref> NUL [<oldvalue>] NUL
 	option SP <opt> NUL
 
-Lines of any other format or a repeated <ref> produce an error.
-Command meanings are:
+In this format, use 40 "0" to specify a zero value, and use the empty
+string to specify a missing value.
+
+In either format, values can be specified in any form that Git
+recognizes as an object name.  Commands in any other format or a
+repeated <ref> produce an error.  Command meanings are:
 
 update::
 	Set <ref> to <newvalue> after verifying <oldvalue>, if given.
@@ -102,9 +111,6 @@ option::
 	The only valid option is `no-deref` to avoid dereferencing
 	a symbolic ref.
 
-Use 40 "0" or the empty string to specify a zero value, except that
-with `-z` an empty <oldvalue> is considered missing.
-
 If all <ref>s can be locked with matching <oldvalue>s
 simultaneously, all modifications are performed.  Otherwise, no
 modifications are performed.  Note that while each individual
diff --git a/builtin/update-ref.c b/builtin/update-ref.c
index c61120faf7..6f3b9095cf 100644
--- a/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@ -154,6 +154,8 @@ static int parse_next_sha1(struct strbuf *input, const char **next,
 				goto invalid;
 		} else if (flags & PARSE_SHA1_ALLOW_EMPTY) {
 			/* With -z, treat an empty value as all zeros: */
+			warning("%s %s: missing <newvalue>, treating as zero",
+				command, refname);
 			hashclr(sha1);
 		} else {
 			/*
diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
index 15f5bfd168..2d61cceb4c 100755
--- a/t/t1400-update-ref.sh
+++ b/t/t1400-update-ref.sh
@@ -730,10 +730,11 @@ test_expect_success 'stdin -z fails update with bad ref name' '
 	grep "fatal: invalid ref format: ~a" err
 '
 
-test_expect_success 'stdin -z treats empty new value as zeros' '
+test_expect_success 'stdin -z emits warning with empty new value' '
 	git update-ref $a $m &&
 	printf $F "update $a" "" "" >stdin &&
-	git update-ref -z --stdin <stdin &&
+	git update-ref -z --stdin <stdin 2>err &&
+	grep "warning: update $a: missing <newvalue>, treating as zero" err &&
 	test_must_fail git rev-parse --verify -q $a
 '