diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 625f83b22667fb3039ccc8981dea9b1b54a9b7d3..daa0125bc0891e723f0a60958d68002af8641c86 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,4 +1,5 @@
-image: "rust:latest"
+# WARNING: update `rust-toolchain.toml` as well
+image: "rust:1.78"
 
 stages:
   - fmt
diff --git a/rust-toolchain.toml b/rust-toolchain.toml
index 2c00fd6a0f52661ba156474e2b0b40e5f432a5ad..f8eeac88483e38e3a9e6a41bf3a80cb1b6f078ec 100644
--- a/rust-toolchain.toml
+++ b/rust-toolchain.toml
@@ -1,4 +1,5 @@
 [toolchain]
 profile = "minimal"
+# WARNING: update `.gitlab-ci.yml` as well
 channel = "1.78"
 components = ["rustfmt", "clippy", "rust-analyzer"]