go: errors parsing go.mod: unexpected input character '\x1f'

Hey there porting experts,

I am in the process of migrating the sysutils/loki port to 
USES=go:modules instead of the quite cumbersome GH_TUPLE shenanigans.

Unfortunately, I am stuck in the go.mod parsing:

# make makesum
===>  License AGPLv3 accepted by the user
===>  License AGPLv3 accepted by the user
===>   grafana-loki-2.9.1 depends on file: /usr/local/sbin/pkg - found
===>   grafana-loki-2.9.1 depends on file: /usr/local/bin/go120 - found
===>   grafana-loki-2.9.1 depends on package: ca_root_nss>0 - found
===> Fetching all distfiles required by grafana-loki-2.9.1 for building
===> Fetching github.com/grafana/loki dependencies
go: errors parsing go.mod:
/usr/ports/distfiles/go/sysutils_loki/grafana-loki-v2.9.1_GH0/go.mod:1: 
unexpected input character '\x1f'
*** Error code 1

Details about my environment below. What am I missing?

In case it helps, I am running on the latest FreeBSD release and latest 
ports installed on ARM64:

FreeBSD freebsd 13.2-RELEASE FreeBSD 13.2-RELEASE 
releng/13.2-n254617-525ecfdad597 GENERIC arm64

This is the (reduced) Makefile I am using that reproduces the issue:

PORTNAME=    loki
DISTVERSIONPREFIX=    v
DISTVERSION=    2.9.1
CATEGORIES=    sysutils
PKGNAMEPREFIX=    grafana-

USES=        go:modules cpe
CPE_VENDOR=    grafana
USE_GITHUB=    yes

USE_RC_SUBR=    loki

GH_ACCOUNT=    grafana
GO_MODULE=    github.com/grafana/loki
GO_TARGET=    ./cmd/loki ./cmd/loki-canary ./cmd/logcli 
./clients/cmd/promtail

.include <bsd.port.mk>

Best,
Christopher