diff --git a/src/compiler/moduleNameResolver.ts b/src/compiler/moduleNameResolver.ts
index f93f2d7eb64..b3ff0951166 100644
--- a/src/compiler/moduleNameResolver.ts
+++ b/src/compiler/moduleNameResolver.ts
@@ -407,18 +407,17 @@ namespace ts {
                 // directory: /a/b/c/d/e
                 // resolvedFileName: /a/b/foo.d.ts
                 const commonPrefix = getCommonPrefix(path, resolvedFileName);
+                if (commonPrefix === undefined) {
+                    return;
+                }
                 let current = path;
-                while (true) {
+                while (current !== commonPrefix) {
                     const parent = getDirectoryPath(current);
                     if (parent === current || directoryPathMap.has(parent)) {
                         break;
                     }
                     directoryPathMap.set(parent, result);
                     current = parent;
-
-                    if (current === commonPrefix) {
-                        break;
-                    }
                 }
             }
 
@@ -434,6 +433,10 @@ namespace ts {
                     i++;
                 }
 
+                if (i === directory.length && resolutionDirectory.length > i && resolutionDirectory[i] === directorySeparator) {
+                    return directory;
+                }
+
                 // find last directory separator before position i
                 const sep = directory.lastIndexOf(directorySeparator, i);
                 if (sep < 0) {