diff --git a/Cargo.toml b/Cargo.toml
index d84b392..156427f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "acme"
 version = "0.1.0"
-edition = "2021"
+edition = "2018"
 license = "Apache-2.0"
 description = "Async pure-Rust ACME client"
 homepage = "https://github.com/InstantDomain/instant-acme"