diff --git a/diff-merges.c b/diff-merges.c
index 146bb50316..ff227368bd 100644
--- a/diff-merges.c
+++ b/diff-merges.c
@@ -2,6 +2,11 @@
 
 #include "revision.h"
 
+typedef void (*diff_merges_setup_func_t)(struct rev_info *);
+static void set_separate(struct rev_info *revs);
+
+static diff_merges_setup_func_t set_to_default = set_separate;
+
 static void suppress(struct rev_info *revs)
 {
 	revs->separate_merges = 0;
@@ -66,6 +71,8 @@ static void set_diff_merges(struct rev_info *revs, const char *optarg)
 		set_combined(revs);
 	else if (!strcmp(optarg, "cc") || !strcmp(optarg, "dense-combined"))
 		set_dense_combined(revs);
+	else if (!strcmp(optarg, "on"))
+		set_to_default(revs);
 	else
 		die(_("unknown value for --diff-merges: %s"), optarg);
 
diff --git a/t/t4013-diff-various.sh b/t/t4013-diff-various.sh
index 6cca8b84a6..26a7b4d19d 100755
--- a/t/t4013-diff-various.sh
+++ b/t/t4013-diff-various.sh
@@ -452,6 +452,14 @@ diff-tree --stat --compact-summary initial mode
 diff-tree -R --stat --compact-summary initial mode
 EOF
 
+test_expect_success 'log --diff-merges=on matches --diff-merges=separate' '
+	git log -p --diff-merges=separate master >result &&
+	process_diffs result >expected &&
+	git log -p --diff-merges=on master >result &&
+	process_diffs result >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'log -S requires an argument' '
 	test_must_fail git log -S
 '