diff --git a/lib/extra-files.js b/lib/extra-files.js
index 3c8196f46..f992e9dfd 100644
--- a/lib/extra-files.js
+++ b/lib/extra-files.js
@@ -16,9 +16,11 @@ const OsHelpers = require('./os-helpers');
  */
 async function collect(requests) {
   const {globby} = await import('globby');
-  const files = await Promise.all(
+  const files2D = await Promise.all(
     requests.map((request) => getFiles(globby, request))
-  ).then((files) => unique(files.flat()));
+  );
+  const flatFiles = files2D.flat();
+  const files = unique(flatFiles);
 
   const filesAndContents = await Promise.all(
     files.map(async (filePath) => {
@@ -32,7 +34,7 @@ async function collect(requests) {
   return filesAndContents.reduce((dict, {path, content}) => {
     dict[path] = content;
     return dict;
-  }, {});
+  }, /** @type {NonElmFiles} */ ({}));
 }
 
 /**
@@ -44,7 +46,9 @@ function getFiles(globby, request) {
   return globby(
     request.files.map(
       (file) =>
-        (file.included ? '' : '!') + OsHelpers.makePathOsAgnostic(file.pattern)
+        `${file.included ? '' : '!'}${OsHelpers.makePathOsAgnostic(
+          file.pattern
+        )}`
     ),
     {
       ignore: request.excludedDirectories,
@@ -58,6 +62,11 @@ function getFiles(globby, request) {
   );
 }
 
+/**
+ * @template T
+ * @param {T[]} array
+ * @returns {T[]}
+ */
 function unique(array) {
   return [...new Set(array)];
 }
diff --git a/tsconfig.no-implicit-any.json b/tsconfig.no-implicit-any.json
index 620481084..68006159d 100644
--- a/tsconfig.no-implicit-any.json
+++ b/tsconfig.no-implicit-any.json
@@ -10,6 +10,7 @@
     "lib/debug.js",
     "lib/elm-communication.js",
     "lib/elm-files.js",
+    "lib/extra-files.js",
     "lib/sync-get.js",
     "lib/sync-get-worker.js",
     "lib/error-message.js",