diff --git a/t/t4068-diff-symmetric.sh b/t/t4068-diff-symmetric-merge-base.sh
similarity index 86%
rename from t/t4068-diff-symmetric.sh
rename to t/t4068-diff-symmetric-merge-base.sh
index 60c506c..bd4cf25 100755
--- a/t/t4068-diff-symmetric.sh
+++ b/t/t4068-diff-symmetric-merge-base.sh
@@ -1,6 +1,6 @@
 #!/bin/sh
 
-test_description='behavior of diff with symmetric-diff setups'
+test_description='behavior of diff with symmetric-diff setups and --merge-base'
 
 . ./test-lib.sh
 
@@ -88,4 +88,13 @@
 	test_i18ngrep "usage" err
 '
 
+test_expect_success 'diff --merge-base with no commits' '
+	test_must_fail git diff --merge-base
+'
+
+test_expect_success 'diff --merge-base with three commits' '
+	test_must_fail git diff --merge-base br1 br2 master 2>err &&
+	test_i18ngrep "usage" err
+'
+
 test_done