diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index e517674..fa6227b 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -66,7 +66,7 @@ check-nightly:
 check-old:
   <<: *check-base
   variables:
-    RUST_VERSION: "1.40.0"
+    RUST_VERSION: "1.39.0"
 
 # Build using Rust stable
 build-x86_64-linux-gnu:
diff --git a/README.md b/README.md
index c3c4aa4..54ed8af 100644
--- a/README.md
+++ b/README.md
@@ -438,7 +438,7 @@ before proceeding:
 ### Build requirements
 - Runtime [requirements](#requirements)
 - [`git`][git]
-- [`rust`][rust] `v1.40` or higher (install using [`rustup`][rustup])
+- [`rust`][rust] `v1.39` or higher (install using [`rustup`][rustup])
 - [OpenSSL][openssl] or [LibreSSL][libressl] libraries and headers must be available
   - Linux:
     - Ubuntu, Debian and derivatives: `apt install build-essential cmake pkg-config libssl-dev`