diff --git a/pkgs/build-support/node/fetch-yarn-deps/index.js b/pkgs/build-support/node/fetch-yarn-deps/index.js
index 04f47362b10d..de2a09ee9041 100755
--- a/pkgs/build-support/node/fetch-yarn-deps/index.js
+++ b/pkgs/build-support/node/fetch-yarn-deps/index.js
@@ -37,7 +37,9 @@ const downloadFileHttps = (fileName, url, expectedHash, hashType = 'sha1') => {
 			res.on('end', () => {
 				file.close()
 				const h = hash.read()
-				if (h != expectedHash) return reject(new Error(`hash mismatch, expected ${expectedHash}, got ${h}`))
+				if (expectedHash === undefined){
+					console.log(`Warning: lockfile url ${url} doesn't end in "#<hash>" to validate against. Downloaded file had hash ${h}.`);
+				} else if (h != expectedHash) return reject(new Error(`hash mismatch, expected ${expectedHash}, got ${h}`))
 				resolve()
 			})
                         res.on('error', e => reject(e))
diff --git a/pkgs/servers/mastodon/default.nix b/pkgs/servers/mastodon/default.nix
index 1547485a0378..d75e148bf1e3 100644
--- a/pkgs/servers/mastodon/default.nix
+++ b/pkgs/servers/mastodon/default.nix
@@ -7,7 +7,8 @@
 , pname ? "mastodon"
 , version ? import ./version.nix
 , srcOverride ? null
-, dependenciesDir ? ./.  # Should contain gemset.nix, yarn.nix and package.json.
+, dependenciesDir ? ./.  # Expected to contain gemset.nix
+, yarnHash ? import ./yarn-hash.nix
 }:
 
 stdenv.mkDerivation rec {
@@ -43,7 +44,7 @@ stdenv.mkDerivation rec {
 
     yarnOfflineCache = fetchYarnDeps {
       yarnLock = "${src}/yarn.lock";
-      sha256 = "sha256-e3rl/WuKXaUdeDEYvo1sSubuIwtBjkbguCYdAijwXOA=";
+      hash = yarnHash;
     };
 
     nativeBuildInputs = [ fixup_yarn_lock nodejs-slim yarn mastodonGems mastodonGems.wrappedRuby brotli ];
diff --git a/pkgs/servers/mastodon/update.sh b/pkgs/servers/mastodon/update.sh
index babc06204851..eafcdd958617 100755
--- a/pkgs/servers/mastodon/update.sh
+++ b/pkgs/servers/mastodon/update.sh
@@ -1,5 +1,5 @@
 #!/usr/bin/env nix-shell
-#! nix-shell -i bash -p yarn2nix bundix coreutils diffutils nix-prefetch-github gnused jq
+#! nix-shell -i bash -p bundix coreutils diffutils nix-prefetch-github gnused jq prefetch-yarn-deps
 set -e
 
 OWNER=mastodon
@@ -77,7 +77,8 @@ trap cleanup EXIT
 
 echo "Fetching source code $REVISION"
 JSON=$(nix-prefetch-github "$OWNER" "$REPO" --rev "$REVISION" 2> $WORK_DIR/nix-prefetch-git.out)
-HASH=$(echo "$JSON" | jq -r .hash)
+HASH="$(echo "$JSON" | jq -r .sha256)"
+HASH="$(nix hash to-sri --type sha256 "$HASH")"
 
 echo "Creating version.nix"
 echo "\"$VERSION\"" | sed 's/^"v/"/' > version.nix
@@ -101,3 +102,8 @@ SOURCE_DIR="$(nix-build --no-out-link -E '(import <nixpkgs> {}).callPackage ./so
 echo "Creating gemset.nix"
 bundix --lockfile="$SOURCE_DIR/Gemfile.lock" --gemfile="$SOURCE_DIR/Gemfile"
 echo "" >> gemset.nix  # Create trailing newline to please EditorConfig checks
+
+echo "Creating yarn-hash.nix"
+YARN_HASH="$(prefetch-yarn-deps "$SOURCE_DIR/yarn.lock")"
+YARN_HASH="$(nix hash to-sri --type sha256 "$YARN_HASH")"
+printf '"%s"\n' "$YARN_HASH" > yarn-hash.nix
diff --git a/pkgs/servers/mastodon/yarn-hash.nix b/pkgs/servers/mastodon/yarn-hash.nix
new file mode 100644
index 000000000000..f0e3e8a27af7
--- /dev/null
+++ b/pkgs/servers/mastodon/yarn-hash.nix
@@ -0,0 +1 @@
+"sha256-e3rl/WuKXaUdeDEYvo1sSubuIwtBjkbguCYdAijwXOA="