diff --git a/api/current.txt b/api/current.txt
index a39bc79a46eeb..b6f5d6435c100 100644
--- a/api/current.txt
+++ b/api/current.txt
@@ -3212,7 +3212,6 @@ package android.app {
     ctor public ActionBar.LayoutParams(int);
     ctor public ActionBar.LayoutParams(android.app.ActionBar.LayoutParams);
     ctor public ActionBar.LayoutParams(android.view.ViewGroup.LayoutParams);
-    ctor public ActionBar.LayoutParams(android.view.ViewGroup.MarginLayoutParams);
     field public int gravity;
   }
 
diff --git a/core/java/android/app/ActionBar.java b/core/java/android/app/ActionBar.java
index 628875f0218e2..5c981802b0dc4 100644
--- a/core/java/android/app/ActionBar.java
+++ b/core/java/android/app/ActionBar.java
@@ -1334,8 +1334,14 @@ public abstract class ActionBar {
             super(source);
         }
 
-        public LayoutParams(MarginLayoutParams source) {
-            super(source);
-        }
+        /*
+         * Note for framework developers:
+         *
+         * You might notice that ActionBar.LayoutParams is missing a constructor overload
+         * for MarginLayoutParams. While it may seem like a good idea to add one, at this
+         * point it's dangerous for source compatibility. Upon building against a new
+         * version of the SDK an app can end up statically linking to the new MarginLayoutParams
+         * overload, causing a crash when running on older platform versions with no other changes.
+         */
     }
 }
diff --git a/core/java/android/view/ViewGroup.java b/core/java/android/view/ViewGroup.java
index 28e8bd63191d3..36e5996e5183c 100644
--- a/core/java/android/view/ViewGroup.java
+++ b/core/java/android/view/ViewGroup.java
@@ -6475,6 +6475,20 @@ public abstract class ViewGroup extends View implements ViewParent, ViewManager
             mMarginFlags &= ~RTL_COMPATIBILITY_MODE_MASK;
         }
 
+        /**
+         * @hide Used internally.
+         */
+        public final void copyMarginsFrom(MarginLayoutParams source) {
+            this.leftMargin = source.leftMargin;
+            this.topMargin = source.topMargin;
+            this.rightMargin = source.rightMargin;
+            this.bottomMargin = source.bottomMargin;
+            this.startMargin = source.startMargin;
+            this.endMargin = source.endMargin;
+
+            this.mMarginFlags = source.mMarginFlags;
+        }
+
         /**
          * Sets the margins, in pixels. A call to {@link android.view.View#requestLayout()} needs
          * to be done so that the new margins are taken into account. Left and right margins may be
diff --git a/core/java/android/widget/Toolbar.java b/core/java/android/widget/Toolbar.java
index 122df2cdd4977..d140c82b38652 100644
--- a/core/java/android/widget/Toolbar.java
+++ b/core/java/android/widget/Toolbar.java
@@ -1585,6 +1585,10 @@ public class Toolbar extends ViewGroup {
     /**
      * Layout information for child views of Toolbars.
      *
+     * <p>Toolbar.LayoutParams extends ActionBar.LayoutParams for compatibility with existing
+     * ActionBar API. See {@link android.app.Activity#setActionBar(Toolbar) Activity.setActionBar}
+     * for more info on how to use a Toolbar as your Activity's ActionBar.</p>
+     *
      * @attr ref android.R.styleable#Toolbar_LayoutParams_layout_gravity
      */
     public static class LayoutParams extends ActionBar.LayoutParams {
@@ -1624,6 +1628,9 @@ public class Toolbar extends ViewGroup {
 
         public LayoutParams(MarginLayoutParams source) {
             super(source);
+            // ActionBar.LayoutParams doesn't have a MarginLayoutParams constructor.
+            // Fake it here and copy over the relevant data.
+            copyMarginsFrom(source);
         }
 
         public LayoutParams(ViewGroup.LayoutParams source) {