diff --git a/Dockerfile b/Dockerfile
index 4c41130..471b44b 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -6,7 +6,7 @@ LABEL maintainer="Robert Kaussow <mail@geeklabor.de>" \
     org.label-schema.vendor="Robert Kaussow" \
     org.label-schema.schema-version="1.0"
 
-ARG FRESHRSS_VERSION="${FRESHRSS_VERSION:-master}"
+ARG FRESHRSS_VERSION="${FRESHRSS_VERSION:=master}"
 ARG FRESHRSS_TARBALL=https://github.com/FreshRSS/FreshRSS/archive/${FRESHRSS_VERSION}.tar.gz
 
 RUN apk --update add --virtual .build-deps tar curl && \