diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index 0e142debedcb0..1c85acfb80e3f 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -189,10 +189,7 @@ namespace Harness.LanguageService {
         getCancellationToken() { return this.cancellationToken; }
         getDirectories(path: string): string[] {
             const dir = this.virtualFileSystem.traversePath(path);
-            if (dir && dir.isDirectory()) {
-                return ts.map((<Utils.VirtualDirectory>dir).getDirectories(), (d) => ts.combinePaths(path, d.name));
-            }
-            return [];
+            return dir && dir.isDirectory() ? dir.getDirectories().map(d => d.name) : [];
         }
         getCurrentDirectory(): string { return virtualFileSystemRoot; }
         getDefaultLibFileName(): string { return Harness.Compiler.defaultLibFileName; }
diff --git a/src/services/completions.ts b/src/services/completions.ts
index 60dfa8aaaa2c0..8a28ae88bb695 100644
--- a/src/services/completions.ts
+++ b/src/services/completions.ts
@@ -359,7 +359,7 @@ namespace ts.Completions {
                 //      import x = require("/*completion position*/");
                 //      var y = require("/*completion position*/");
                 //      export * from "/*completion position*/";
-                return pathCompletionsInfo(PathCompletions.getStringLiteralCompletionsFromModuleNames(sourceFile, node as StringLiteral, compilerOptions, host, typeChecker));
+                return pathCompletionsInfo(PathCompletions.getStringLiteralCompletionsFromModuleNames(sourceFile, node, compilerOptions, host, typeChecker));
 
             default:
                 return fromContextualType();
diff --git a/src/services/pathCompletions.ts b/src/services/pathCompletions.ts
index 8840919c32963..d27417ec521dd 100644
--- a/src/services/pathCompletions.ts
+++ b/src/services/pathCompletions.ts
@@ -6,7 +6,7 @@ namespace ts.Completions.PathCompletions {
         const scriptPath = node.getSourceFile().path;
         const scriptDirectory = getDirectoryPath(scriptPath);
 
-        const span = getDirectoryFragmentTextSpan((<StringLiteral>node).text, node.getStart(sourceFile) + 1);
+        const span = getDirectoryFragmentTextSpan(node.text, node.getStart(sourceFile) + 1);
         if (isPathRelativeToScript(literalValue) || isRootedDiskPath(literalValue)) {
             const extensions = getSupportedExtensions(compilerOptions);
             if (compilerOptions.rootDirs) {
@@ -226,7 +226,8 @@ namespace ts.Completions.PathCompletions {
         const includeGlob = normalizedSuffix ? "**/*" : "./*";
 
         const matches = tryReadDirectory(host, baseDirectory, fileExtensions, /*exclude*/ undefined, [includeGlob]);
-        const directories = tryGetDirectories(host, baseDirectory);
+        const directories = tryGetDirectories(host, baseDirectory).map(d => combinePaths(baseDirectory, d));
+
         // Trim away prefix and suffix
         return mapDefined(concatenate(matches, directories), match => {
             const normalizedMatch = normalizePath(match);
@@ -476,14 +477,14 @@ namespace ts.Completions.PathCompletions {
     const nodeModulesDependencyKeys = ["dependencies", "devDependencies", "peerDependencies", "optionalDependencies"];
 
     function tryGetDirectories(host: LanguageServiceHost, directoryName: string): string[] {
-        return tryIOAndConsumeErrors(host, host.getDirectories, directoryName);
+        return tryIOAndConsumeErrors(host, host.getDirectories, directoryName) || [];
     }
 
-    function tryReadDirectory(host: LanguageServiceHost, path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>): string[] | undefined {
+    function tryReadDirectory(host: LanguageServiceHost, path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>): string[] | undefined | undefined {
         return tryIOAndConsumeErrors(host, host.readDirectory, path, extensions, exclude, include);
     }
 
-    function tryReadFile(host: LanguageServiceHost, path: string): string {
+    function tryReadFile(host: LanguageServiceHost, path: string): string | undefined {
         return tryIOAndConsumeErrors(host, host.readFile, path);
     }
 
diff --git a/src/services/types.ts b/src/services/types.ts
index 4ebd4a3e76a1e..594484e1ea733 100644
--- a/src/services/types.ts
+++ b/src/services/types.ts
@@ -200,8 +200,8 @@ namespace ts {
         /* @internal */ hasChangedAutomaticTypeDirectiveNames?: boolean;
 
         /*
-         * getDirectories is also required for full import and type reference completions. Without it defined, certain
-         * completions will not be provided
+         * Required for full import and type reference completions.
+         * These should be unprefixed names. E.g. `getDirectories("/foo/bar")` should return `["a", "b"]`, not `["/foo/bar/a", "/foo/bar/b"]`.
          */
         getDirectories?(directoryName: string): string[];
 
diff --git a/tests/cases/fourslash/completionsPaths_pathMapping.ts b/tests/cases/fourslash/completionsPaths_pathMapping.ts
index 7a6283bce3a8c..80e0fa26d9fc9 100644
--- a/tests/cases/fourslash/completionsPaths_pathMapping.ts
+++ b/tests/cases/fourslash/completionsPaths_pathMapping.ts
@@ -20,8 +20,4 @@
 ////}
 
 const [replacementSpan] = test.ranges();
-verify.completionsAt("", [
-    { name: "a", replacementSpan },
-    { name: "b", replacementSpan },
-    { name: "dir", replacementSpan },
-]);
+verify.completionsAt("", ["a", "b", "dir"].map(name => ({ name, replacementSpan })));