diff --git a/src/cdk/bidi/dir.ts b/src/cdk/bidi/dir.ts
index f5fe1230a839..dbf246361376 100644
--- a/src/cdk/bidi/dir.ts
+++ b/src/cdk/bidi/dir.ts
@@ -41,9 +41,9 @@ export class Dir implements Directionality, AfterContentInit, OnDestroy {
   /** @docs-private */
   @Input()
   get dir(): Direction { return this._dir; }
-  set dir(v: Direction) {
+  set dir(value: Direction) {
     const old = this._dir;
-    this._dir = v;
+    this._dir = (value === 'ltr' || value === 'rtl') ? value : 'ltr';
     if (old !== this._dir && this._isInitialized) {
       this.change.emit(this._dir);
     }
diff --git a/src/cdk/bidi/directionality.spec.ts b/src/cdk/bidi/directionality.spec.ts
index 7451d3247ba9..89414a81859b 100644
--- a/src/cdk/bidi/directionality.spec.ts
+++ b/src/cdk/bidi/directionality.spec.ts
@@ -1,7 +1,7 @@
 import {async, fakeAsync, TestBed} from '@angular/core/testing';
-import {Component} from '@angular/core';
+import {Component, ViewChild} from '@angular/core';
 import {By} from '@angular/platform-browser';
-import {BidiModule, Directionality, Direction, DIR_DOCUMENT} from './index';
+import {BidiModule, Directionality, Dir, Direction, DIR_DOCUMENT} from './index';
 
 describe('Directionality', () => {
   let fakeDocument: FakeDocument;
@@ -55,6 +55,15 @@ describe('Directionality', () => {
       subscription.unsubscribe();
     });
 
+    it('should default to ltr if an invalid direction is set on the body', () => {
+      fakeDocument.body.dir = 'not-valid';
+
+      const fixture = TestBed.createComponent(InjectsDirectionality);
+      const testComponent = fixture.debugElement.componentInstance;
+
+      expect(testComponent.dir.value).toBe('ltr');
+    });
+
   });
 
   describe('Dir directive', () => {
@@ -103,6 +112,17 @@ describe('Directionality', () => {
       subscription.unsubscribe();
     }));
 
+    it('should default to ltr if an invalid value is passed in', () => {
+      const fixture = TestBed.createComponent(ElementWithDir);
+
+      fixture.detectChanges();
+      expect(fixture.componentInstance.dir.value).toBe('rtl');
+
+      fixture.componentInstance.direction = 'not-valid';
+      fixture.detectChanges();
+      expect(fixture.componentInstance.dir.value).toBe('ltr');
+    });
+
   });
 });
 
@@ -115,6 +135,7 @@ describe('Directionality', () => {
   `
 })
 class ElementWithDir {
+  @ViewChild(Dir) dir: Dir;
   direction = 'rtl';
   changeCount = 0;
 }
diff --git a/src/cdk/bidi/directionality.ts b/src/cdk/bidi/directionality.ts
index 88a5c068d9eb..1c95de0109f8 100644
--- a/src/cdk/bidi/directionality.ts
+++ b/src/cdk/bidi/directionality.ts
@@ -33,7 +33,8 @@ export class Directionality implements OnDestroy {
       // but getComputedStyle return either "ltr" or "rtl". avoiding getComputedStyle for now
       const bodyDir = _document.body ? _document.body.dir : null;
       const htmlDir = _document.documentElement ? _document.documentElement.dir : null;
-      this.value = (bodyDir || htmlDir || 'ltr') as Direction;
+      const value = bodyDir || htmlDir;
+      this.value = (value === 'ltr' || value === 'rtl') ? value : 'ltr';
     }
   }