importer is undefined for entry modules

https://github.com/rollup/rollup/blob/master/src/Bundle.js#L116

types/rollup/index.d.ts

@@ -135,7 +135,7 @@ export interface Plugin {
 	 * Returning null or undefined defers to other resolveId functions (and eventually the default resolution behavior);
 	 * returning any other falsy value signals that importee should be treated as an external module and not included in the bundle.
 	 */
-	resolveId?(importee: string, importer: string): string | null | undefined | false | 0 | ''
+	resolveId?(importee: string, importer: string | undefined): string | null | undefined | false | 0 | ''
 	/** A module transformer function */
 	transform?(this: TransformContext, source: string, id: string): string | { code: string, map: SourceMap }
 	/** A bundle transformer function */

types/rollup/rollup-tests.ts

@@ -5,6 +5,12 @@ declare const console: { log(...messages: any[]): void, warn(message: string): v
 let cache: Bundle | undefined
 const plugin: Plugin = {
     name: 'test-plugin',
+    resolveId(importee, importer) {
+        if (importer === undefined) {
+            return 'custom/entry/point.js'
+        }
+        return importer.trim()
+    },
     transform(source, id) {
         if (id === 'rxjs') {
             this.error(new Error(`Don't import this directly`))