diff --git a/src/index.ts b/src/index.ts
index 48d2c72ee..f9500ce5a 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -81,7 +81,7 @@ class MemoryCache {
   fileContents = new Map<string, string>()
   fileVersions = new Map<string, number>()
 
-  constructor (rootFileNames: string[] = []) {
+  constructor (public rootFileNames: string[] = []) {
     for (const fileName of rootFileNames) this.fileVersions.set(fileName, 1)
   }
 }
@@ -293,7 +293,7 @@ export function register (opts: Options = {}): Register {
 
     // Create the compiler host for type checking.
     const serviceHost: _ts.LanguageServiceHost = {
-      getScriptFileNames: () => Array.from(memoryCache.fileVersions.keys()),
+      getScriptFileNames: () => memoryCache.rootFileNames,
       getScriptVersion: (fileName: string) => {
         const version = memoryCache.fileVersions.get(fileName)
         return version === undefined ? '' : version.toString()
@@ -332,6 +332,9 @@ export function register (opts: Options = {}): Register {
     const updateMemoryCache = (contents: string, fileName: string) => {
       const fileVersion = memoryCache.fileVersions.get(fileName) || 0
 
+      // Add to `rootFiles` when discovered for the first time.
+      if (fileVersion === 0) memoryCache.rootFileNames.push(fileName)
+
       // Avoid incrementing cache when nothing has changed.
       if (memoryCache.fileContents.get(fileName) === contents) return