diff --git a/pkgs/build-support/node/fetch-yarn-deps/index.js b/pkgs/build-support/node/fetch-yarn-deps/index.js
index de2a09ee9041..8dde3ef39085 100755
--- a/pkgs/build-support/node/fetch-yarn-deps/index.js
+++ b/pkgs/build-support/node/fetch-yarn-deps/index.js
@@ -88,10 +88,17 @@ const isGitUrl = pattern => {
 }
 
 const downloadPkg = (pkg, verbose) => {
-	const [ name, spec ] = pkg.key.split('@', 2);
-	if (spec.startsWith('file:')) {
-		console.info(`ignoring relative file:path dependency "${spec}"`)
+	const fileMarker = '@file:'
+	const split = pkg.key.split(fileMarker)
+	if (split.length == 2) {
+		console.info(`ignoring lockfile entry "${split[0]}" which points at path "${split[1]}"`)
 		return
+	} else if (split.length > 2) {
+		throw new Error(`The lockfile entry key "${pkg.key}" contains "${fileMarker}" more than once. Processing is not implemented.`)
+	}
+
+	if (pkg.resolved === undefined) {
+		throw new Error(`The lockfile entry with key "${pkg.key}" cannot be downloaded because it is missing the "resolved" attribute, which should contain the URL to download from. The lockfile might be invalid.`)
 	}
 
 	const [ url, hash ] = pkg.resolved.split('#')
diff --git a/pkgs/build-support/node/fetch-yarn-deps/tests/default.nix b/pkgs/build-support/node/fetch-yarn-deps/tests/default.nix
index 8ffe103a9548..8057d05ba72c 100644
--- a/pkgs/build-support/node/fetch-yarn-deps/tests/default.nix
+++ b/pkgs/build-support/node/fetch-yarn-deps/tests/default.nix
@@ -1,6 +1,10 @@
 { testers, fetchYarnDeps, ... }:
 
 {
+  file = testers.invalidateFetcherByDrvHash fetchYarnDeps {
+    yarnLock = ./file.lock;
+    sha256 = "sha256-BPuyQVCbdpFL/iRhmarwWAmWO2NodlVCOY9JU+4pfa4=";
+  };
   simple = testers.invalidateFetcherByDrvHash fetchYarnDeps {
     yarnLock = ./simple.lock;
     sha256 = "sha256-FRrt8BixleILmFB2ZV8RgPNLqgS+dlH5nWoPgeaaNQ8=";
diff --git a/pkgs/build-support/node/fetch-yarn-deps/tests/file.lock b/pkgs/build-support/node/fetch-yarn-deps/tests/file.lock
new file mode 100644
index 000000000000..4881d83a7de9
--- /dev/null
+++ b/pkgs/build-support/node/fetch-yarn-deps/tests/file.lock
@@ -0,0 +1,9 @@
+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+
+"@org/somepack@file:vendor/orgpacks/somepack/assets":
+  version "1.0.0"
+
+"otherpack@file:vendor/otherpack":
+  version "1.0.0"