diff --git a/builtin/am.c b/builtin/am.c
index 3d98e52085..06b824518f 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -708,6 +708,7 @@ static int split_mail_mbox(struct am_state *state, const char **paths,
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
 	struct strbuf last = STRBUF_INIT;
+	int ret;
 
 	cp.git_cmd = 1;
 	argv_array_push(&cp.args, "mailsplit");
@@ -721,13 +722,16 @@ static int split_mail_mbox(struct am_state *state, const char **paths,
 	argv_array_push(&cp.args, "--");
 	argv_array_pushv(&cp.args, paths);
 
-	if (capture_command(&cp, &last, 8))
-		return -1;
+	ret = capture_command(&cp, &last, 8);
+	if (ret)
+		goto exit;
 
 	state->cur = 1;
 	state->last = strtol(last.buf, NULL, 10);
 
-	return 0;
+exit:
+	strbuf_release(&last);
+	return ret ? -1 : 0;
 }
 
 /**