diff --git a/dir.c b/dir.c
index b48e19dc0..602282bd1 100644
--- a/dir.c
+++ b/dir.c
@@ -24,8 +24,9 @@ int common_prefix(const char **pathspec)
 	prefix = slash - path + 1;
 	while ((next = *++pathspec) != NULL) {
 		int len = strlen(next);
-		if (len >= prefix && !memcmp(path, next, len))
+		if (len >= prefix && !memcmp(path, next, prefix))
 			continue;
+		len = prefix - 1;
 		for (;;) {
 			if (!len)
 				return 0;
diff --git a/t/t3700-add.sh b/t/t3700-add.sh
index 08e035220..ad8cc7d4a 100755
--- a/t/t3700-add.sh
+++ b/t/t3700-add.sh
@@ -104,4 +104,10 @@ test_expect_success 'add ignored ones with -f' '
 	git-ls-files --error-unmatch d.ig/d.if d.ig/d.ig
 '
 
+mkdir 1 1/2 1/3
+touch 1/2/a 1/3/b 1/2/c
+test_expect_success 'check correct prefix detection' '
+	git add 1/2/a 1/3/b 1/2/c
+'
+
 test_done