diff --git a/README.md b/README.md
index 2181d5c42cd..e03d31d3675 100644
--- a/README.md
+++ b/README.md
@@ -77,6 +77,7 @@ Caddy binaries have no dependencies and are available for every platform. Get Ca
 
 To build from source you need **[Git](https://git-scm.com/downloads)** and **[Go](https://golang.org/doc/install)** (1.12 or newer). Follow these instruction for fast building:
 
+- Set the transitional environment variable `GO111MODULE` to `on`
 - Get the source with `go get github.com/mholt/caddy/caddy` and then run `go get github.com/caddyserver/builds`
 - Now `cd $GOPATH/src/github.com/mholt/caddy/caddy` and run `go run build.go`