diff --git a/src/compiler/moduleSpecifiers.ts b/src/compiler/moduleSpecifiers.ts
index c17dfeabcf1..db0b49f4436 100644
--- a/src/compiler/moduleSpecifiers.ts
+++ b/src/compiler/moduleSpecifiers.ts
@@ -854,8 +854,8 @@ namespace ts.moduleSpecifiers {
 
     function getPathRelativeToRootDirs(path: string, rootDirs: readonly string[], getCanonicalFileName: GetCanonicalFileName): string | undefined {
         return firstDefined(rootDirs, rootDir => {
-            const relativePath = getRelativePathIfInDirectory(path, rootDir, getCanonicalFileName)!; // TODO: GH#18217
-            return isPathRelativeToParent(relativePath) ? undefined : relativePath;
+            const relativePath = getRelativePathIfInDirectory(path, rootDir, getCanonicalFileName);
+            return relativePath !== undefined && isPathRelativeToParent(relativePath) ? undefined : relativePath;
         });
     }
 
diff --git a/tests/cases/fourslash/autoImportsWithRootDirsAndRootedPath01.ts b/tests/cases/fourslash/autoImportsWithRootDirsAndRootedPath01.ts
new file mode 100644
index 00000000000..08073349cb1
--- /dev/null
+++ b/tests/cases/fourslash/autoImportsWithRootDirsAndRootedPath01.ts
@@ -0,0 +1,24 @@
+/// <reference path="./fourslash.ts" />
+
+// @Filename: /dir/foo.ts
+//// export function foo() {}
+
+// @Filename: /dir/bar.ts
+//// /*$*/
+
+// @Filename: /dir/tsconfig.json
+////{
+////    "compilerOptions": {
+////        "module": "amd",
+////        "moduleResolution": "classic",
+////        "rootDirs": ["D:/"]
+////    }
+////}
+
+goTo.marker("$");
+verify.completions({
+    preferences: {
+        includeCompletionsForModuleExports: true,
+        allowIncompleteCompletions: true,
+    }
+});