diff --git a/certifi.go b/certifi.go
index a152a0d..4873602 100644
--- a/certifi.go
+++ b/certifi.go
@@ -1,10 +1,10 @@
 // Code generated by go generate; DO NOT EDIT.
-// 2019-04-09 17:53:46.117677 -0700 PDT m=+1.272386477
+// 2019-04-15 13:37:32.862635 +0200 CEST m=+1.088810069
 // https://mkcert.org/generate/
 
 package gocertifi
 
-//go:generate go run gen.go
+//go:generate go run gen.go "https://mkcert.org/generate/"
 
 import "crypto/x509"
 
diff --git a/gen.go b/gen.go
index 0a26544..fe72046 100644
--- a/gen.go
+++ b/gen.go
@@ -8,12 +8,17 @@ import (
 	"log"
 	"net/http"
 	"os"
+	"strings"
 	"text/template"
 	"time"
 )
 
 func main() {
-	const url = "https://mkcert.org/generate/"
+	if len(os.Args) != 2 || !strings.HasPrefix(os.Args[1], "https://") {
+		log.Fatal("usage: go run gen.go <url>")
+	}
+	url := os.Args[1]
+
 	resp, err := http.Get(url)
 	if err != nil {
 		log.Fatal(err)
@@ -53,7 +58,7 @@ var tmpl = template.Must(template.New("").Parse(`// Code generated by go generat
 
 package gocertifi
 
-//go:generate go run gen.go
+//go:generate go run gen.go "{{ .URL }}"
 
 import "crypto/x509"