diff --git a/git-bisect.sh b/git-bisect.sh
index b2186a8627..20f6dd5625 100755
--- a/git-bisect.sh
+++ b/git-bisect.sh
@@ -59,6 +59,39 @@ bisect_autostart() {
 }
 
 bisect_start() {
+	#
+	# Check for one bad and then some good revisions.
+	#
+	has_double_dash=0
+	for arg; do
+	    case "$arg" in --) has_double_dash=1; break ;; esac
+	done
+	orig_args=$(git rev-parse --sq-quote "$@")
+	bad_seen=0
+	eval=''
+	while [ $# -gt 0 ]; do
+	    arg="$1"
+	    case "$arg" in
+	    --)
+		shift
+		break
+		;;
+	    *)
+		rev=$(git rev-parse -q --verify "$arg^{commit}") || {
+		    test $has_double_dash -eq 1 &&
+			die "$(eval_gettext "'\$arg' does not appear to be a valid revision")"
+		    break
+		}
+		case $bad_seen in
+		0) state='bad' ; bad_seen=1 ;;
+		*) state='good' ;;
+		esac
+		eval="$eval bisect_write '$state' '$rev' 'nolog'; "
+		shift
+		;;
+	    esac
+	done
+
 	#
 	# Verify HEAD.
 	#
@@ -97,39 +130,6 @@ bisect_start() {
 	#
 	bisect_clean_state || exit
 
-	#
-	# Check for one bad and then some good revisions.
-	#
-	has_double_dash=0
-	for arg; do
-	    case "$arg" in --) has_double_dash=1; break ;; esac
-	done
-	orig_args=$(git rev-parse --sq-quote "$@")
-	bad_seen=0
-	eval=''
-	while [ $# -gt 0 ]; do
-	    arg="$1"
-	    case "$arg" in
-	    --)
-		shift
-		break
-		;;
-	    *)
-		rev=$(git rev-parse -q --verify "$arg^{commit}") || {
-		    test $has_double_dash -eq 1 &&
-			die "$(eval_gettext "'\$arg' does not appear to be a valid revision")"
-		    break
-		}
-		case $bad_seen in
-		0) state='bad' ; bad_seen=1 ;;
-		*) state='good' ;;
-		esac
-		eval="$eval bisect_write '$state' '$rev' 'nolog'; "
-		shift
-		;;
-	    esac
-	done
-
 	#
 	# Change state.
 	# In case of mistaken revs or checkout error, or signals received,
diff --git a/t/t6030-bisect-porcelain.sh b/t/t6030-bisect-porcelain.sh
index b5063b6fe6..b3d1b1430b 100755
--- a/t/t6030-bisect-porcelain.sh
+++ b/t/t6030-bisect-porcelain.sh
@@ -138,15 +138,23 @@ test_expect_success 'bisect start: back in good branch' '
 	grep "* other" branch.output > /dev/null
 '
 
-test_expect_success 'bisect start: no ".git/BISECT_START" if junk rev' '
-	git bisect start $HASH4 $HASH1 -- &&
-	git bisect good &&
+test_expect_success 'bisect start: no ".git/BISECT_START" created if junk rev' '
+	git bisect reset &&
 	test_must_fail git bisect start $HASH4 foo -- &&
 	git branch > branch.output &&
 	grep "* other" branch.output > /dev/null &&
 	test_must_fail test -e .git/BISECT_START
 '
 
+test_expect_success 'bisect start: existing ".git/BISECT_START" not modified if junk rev' '
+	git bisect start $HASH4 $HASH1 -- &&
+	git bisect good &&
+	cp .git/BISECT_START saved &&
+	test_must_fail git bisect start $HASH4 foo -- &&
+	git branch > branch.output &&
+	grep "* (no branch)" branch.output > /dev/null &&
+	test_cmp saved .git/BISECT_START
+'
 test_expect_success 'bisect start: no ".git/BISECT_START" if mistaken rev' '
 	git bisect start $HASH4 $HASH1 -- &&
 	git bisect good &&