diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 6435508..5c9a590 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -3314,8 +3314,12 @@ static int module_add(int argc, const char **argv, const char *prefix)
 	else
 		add_data.sm_path = xstrdup(argv[1]);
 
-	if (prefix && *prefix && !is_absolute_path(add_data.sm_path))
-		add_data.sm_path = xstrfmt("%s%s", prefix, add_data.sm_path);
+	if (prefix && *prefix && !is_absolute_path(add_data.sm_path)) {
+		char *sm_path = add_data.sm_path;
+
+		add_data.sm_path = xstrfmt("%s%s", prefix, sm_path);
+		free(sm_path);
+	}
 
 	if (starts_with_dot_dot_slash(add_data.repo) ||
 	    starts_with_dot_slash(add_data.repo)) {