diff --git a/builtin/diff-index.c b/builtin/diff-index.c
index 176fe7ff2b4..cf09559e422 100644
--- a/builtin/diff-index.c
+++ b/builtin/diff-index.c
@@ -2,6 +2,7 @@
 #include "cache.h"
 #include "config.h"
 #include "diff.h"
+#include "diff-merges.h"
 #include "commit.h"
 #include "revision.h"
 #include "builtin.h"
@@ -27,6 +28,12 @@ int cmd_diff_index(int argc, const char **argv, const char *prefix)
 	rev.abbrev = 0;
 	prefix = precompose_argv_prefix(argc, argv, prefix);
 
+	/*
+	 * We need no diff for merges options, and we need to avoid conflict
+	 * with our own meaning of "-m".
+	 */
+	diff_merges_suppress_options_parsing();
+
 	argc = setup_revisions(argc, argv, &rev, NULL);
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
@@ -35,6 +42,8 @@ int cmd_diff_index(int argc, const char **argv, const char *prefix)
 			option |= DIFF_INDEX_CACHED;
 		else if (!strcmp(arg, "--merge-base"))
 			option |= DIFF_INDEX_MERGE_BASE;
+		else if (!strcmp(arg, "-m"))
+			rev.match_missing = 1;
 		else
 			usage(diff_cache_usage);
 	}
diff --git a/diff-merges.c b/diff-merges.c
index f3a9daed7e0..9ca00cdd0cc 100644
--- a/diff-merges.c
+++ b/diff-merges.c
@@ -6,6 +6,7 @@ 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 int suppress_parsing;
 
 static void suppress(struct rev_info *revs)
 {
@@ -30,17 +31,6 @@ static void set_first_parent(struct rev_info *revs)
 	revs->first_parent_merges = 1;
 }
 
-static void set_m(struct rev_info *revs)
-{
-	/*
-	 * To "diff-index", "-m" means "match missing", and to the "log"
-	 * family of commands, it means "show default diff for merges". Set
-	 * both fields appropriately.
-	 */
-	set_to_default(revs);
-	revs->match_missing = 1;
-}
-
 static void set_combined(struct rev_info *revs)
 {
 	suppress(revs);
@@ -101,14 +91,22 @@ int diff_merges_config(const char *value)
 	return 0;
 }
 
+void diff_merges_suppress_options_parsing(void)
+{
+	suppress_parsing = 1;
+}
+
 int diff_merges_parse_opts(struct rev_info *revs, const char **argv)
 {
 	int argcount = 1;
 	const char *optarg;
 	const char *arg = argv[0];
 
+	if (suppress_parsing)
+		return 0;
+
 	if (!strcmp(arg, "-m")) {
-		set_m(revs);
+		set_to_default(revs);
 	} else if (!strcmp(arg, "-c")) {
 		set_combined(revs);
 		revs->combined_imply_patch = 1;
@@ -155,6 +153,9 @@ void diff_merges_set_dense_combined_if_unset(struct rev_info *revs)
 
 void diff_merges_setup_revs(struct rev_info *revs)
 {
+	if (suppress_parsing)
+		return;
+
 	if (revs->combine_merges == 0)
 		revs->dense_combined_merges = 0;
 	if (revs->separate_merges == 0)
diff --git a/diff-merges.h b/diff-merges.h
index 09d9a6c9a4f..b5d57f6563e 100644
--- a/diff-merges.h
+++ b/diff-merges.h
@@ -11,6 +11,8 @@ struct rev_info;
 
 int diff_merges_config(const char *value);
 
+void diff_merges_suppress_options_parsing(void);
+
 int diff_merges_parse_opts(struct rev_info *revs, const char **argv);
 
 void diff_merges_suppress(struct rev_info *revs);