--- a/mobile/android/geckoview/src/androidTest/java/org/mozilla/geckoview/test/GeckoSessionTestRuleTest.kt
+++ b/mobile/android/geckoview/src/androidTest/java/org/mozilla/geckoview/test/GeckoSessionTestRuleTest.kt
@@ -3,16 +3,17 @@
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
 package org.mozilla.geckoview.test
 
 import org.mozilla.geckoview.GeckoSession
 import org.mozilla.geckoview.GeckoSessionSettings
 import org.mozilla.geckoview.test.rule.GeckoSessionTestRule.AssertCalled
 import org.mozilla.geckoview.test.rule.GeckoSessionTestRule.ClosedSessionAtStart
+import org.mozilla.geckoview.test.rule.GeckoSessionTestRule.NullDelegate
 import org.mozilla.geckoview.test.rule.GeckoSessionTestRule.Setting
 import org.mozilla.geckoview.test.rule.GeckoSessionTestRule.TimeoutException
 import org.mozilla.geckoview.test.rule.GeckoSessionTestRule.TimeoutMillis
 import org.mozilla.geckoview.test.rule.GeckoSessionTestRule.WithDisplay
 import org.mozilla.geckoview.test.util.Callbacks
 
 import android.support.test.filters.MediumTest
 import android.support.test.runner.AndroidJUnit4
@@ -71,16 +72,48 @@ class GeckoSessionTestRuleTest : BaseSes
             if (!ifce.isInterface || !ifce.simpleName.endsWith("Delegate")) {
                 continue
             }
             assertThat("Callbacks.All should include interface " + ifce.simpleName,
                        ifce.isInstance(Callbacks.Default), equalTo(true))
         }
     }
 
+    @NullDelegate.List(NullDelegate(GeckoSession.ContentDelegate::class),
+                       NullDelegate(Callbacks.NavigationDelegate::class))
+    @NullDelegate(Callbacks.ScrollDelegate::class)
+    @Test fun nullDelegate() {
+        assertThat("Content delegate should be null",
+                   sessionRule.session.contentDelegate, nullValue())
+        assertThat("Navigation delegate should be null",
+                   sessionRule.session.navigationDelegate, nullValue())
+        assertThat("Scroll delegate should be null",
+                   sessionRule.session.scrollDelegate, nullValue())
+
+        assertThat("Progress delegate should not be null",
+                   sessionRule.session.progressDelegate, notNullValue())
+    }
+
+    @NullDelegate(GeckoSession.ProgressDelegate::class)
+    @ClosedSessionAtStart
+    @Test fun nullDelegate_closed() {
+        assertThat("Progress delegate should be null",
+                   sessionRule.session.progressDelegate, nullValue())
+    }
+
+    @Test(expected = AssertionError::class)
+    @NullDelegate(GeckoSession.ProgressDelegate::class)
+    @ClosedSessionAtStart
+    fun nullDelegate_requireProgressOnOpen() {
+        assertThat("Progress delegate should be null",
+                   sessionRule.session.progressDelegate, nullValue())
+
+        sessionRule.session.open()
+    }
+
     @Test fun waitForPageStop() {
         sessionRule.session.loadTestPath(HELLO_HTML_PATH)
         sessionRule.waitForPageStop()
 
         var counter = 0
 
         sessionRule.forCallbacksDuringWait(object : Callbacks.ProgressDelegate {
             override fun onPageStop(session: GeckoSession, success: Boolean) {
@@ -109,16 +142,25 @@ class GeckoSessionTestRuleTest : BaseSes
             override fun onPageStop(session: GeckoSession, success: Boolean) {
                 counter++
             }
         })
 
         assertThat("Callback count should be correct", counter, equalTo(2))
     }
 
+    @Test(expected = AssertionError::class)
+    @NullDelegate(GeckoSession.ProgressDelegate::class)
+    @ClosedSessionAtStart
+    fun waitForPageStops_throwOnNullDelegate() {
+        sessionRule.session.open(sessionRule.runtime) // Avoid waiting for initial load
+        sessionRule.session.reload()
+        sessionRule.session.waitForPageStops(2)
+    }
+
     @Test fun waitUntilCalled_anyInterfaceMethod() {
         sessionRule.session.loadTestPath(HELLO_HTML_PATH)
         sessionRule.waitUntilCalled(GeckoSession.ProgressDelegate::class)
 
         var counter = 0
 
         sessionRule.forCallbacksDuringWait(object : Callbacks.ProgressDelegate {
             override fun onPageStart(session: GeckoSession, url: String) {
@@ -164,16 +206,29 @@ class GeckoSessionTestRuleTest : BaseSes
         sessionRule.waitUntilCalled(CharSequence::class)
     }
 
     fun waitUntilCalled_notThrowOnCallbackInterface() {
         sessionRule.session.loadTestPath(HELLO_HTML_PATH)
         sessionRule.waitUntilCalled(Callbacks.ProgressDelegate::class)
     }
 
+    @Test(expected = AssertionError::class)
+    @NullDelegate(GeckoSession.ScrollDelegate::class)
+    fun waitUntilCalled_throwOnNullDelegateInterface() {
+        sessionRule.session.reload()
+        sessionRule.session.waitUntilCalled(Callbacks.All::class)
+    }
+
+    @NullDelegate(GeckoSession.ScrollDelegate::class)
+    @Test fun waitUntilCalled_notThrowOnNonNullDelegateMethod() {
+        sessionRule.session.reload()
+        sessionRule.session.waitUntilCalled(Callbacks.All::class, "onPageStop")
+    }
+
     @Test fun waitUntilCalled_anyObjectMethod() {
         sessionRule.session.loadTestPath(HELLO_HTML_PATH)
 
         var counter = 0
 
         sessionRule.waitUntilCalled(object : Callbacks.ProgressDelegate {
             override fun onPageStart(session: GeckoSession, url: String) {
                 counter++
@@ -207,16 +262,37 @@ class GeckoSessionTestRuleTest : BaseSes
             override fun onPageStop(session: GeckoSession, success: Boolean) {
                 counter++
             }
         })
 
         assertThat("Callback count should be correct", counter, equalTo(2))
     }
 
+    @Test(expected = AssertionError::class)
+    @NullDelegate(GeckoSession.ScrollDelegate::class)
+    fun waitUntilCalled_throwOnNullDelegateObject() {
+        sessionRule.session.reload()
+        sessionRule.session.waitUntilCalled(object : Callbacks.All {
+            @AssertCalled
+            override fun onScrollChanged(session: GeckoSession, scrollX: Int, scrollY: Int) {
+            }
+        })
+    }
+
+    @NullDelegate(GeckoSession.ScrollDelegate::class)
+    @Test fun waitUntilCalled_notThrowOnNonNullDelegateObject() {
+        sessionRule.session.reload()
+        sessionRule.session.waitUntilCalled(object : Callbacks.All {
+            @AssertCalled
+            override fun onPageStop(session: GeckoSession, success: Boolean) {
+            }
+        })
+    }
+
     @Test fun waitUntilCalled_multipleCount() {
         sessionRule.session.loadTestPath(HELLO_HTML_PATH)
         sessionRule.session.reload()
 
         var counter = 0
 
         sessionRule.waitUntilCalled(object : Callbacks.ProgressDelegate {
             @AssertCalled(count = 2)
@@ -540,16 +616,50 @@ class GeckoSessionTestRuleTest : BaseSes
             @AssertCalled
             override fun onPageStop(session: GeckoSession, success: Boolean) {
                 throw IllegalStateException()
             }
         })
     }
 
     @Test(expected = AssertionError::class)
+    @NullDelegate(GeckoSession.ScrollDelegate::class)
+    fun forCallbacksDuringWait_throwOnAnyNullDelegate() {
+        sessionRule.session.reload()
+        sessionRule.session.waitForPageStop()
+
+        sessionRule.session.forCallbacksDuringWait(object : Callbacks.All {})
+    }
+
+    @Test(expected = AssertionError::class)
+    @NullDelegate(GeckoSession.ScrollDelegate::class)
+    fun forCallbacksDuringWait_throwOnSpecificNullDelegate() {
+        sessionRule.session.reload()
+        sessionRule.session.waitForPageStop()
+
+        sessionRule.session.forCallbacksDuringWait(object : Callbacks.All {
+            @AssertCalled
+            override fun onScrollChanged(session: GeckoSession, scrollX: Int, scrollY: Int) {
+            }
+        })
+    }
+
+    @NullDelegate(GeckoSession.ScrollDelegate::class)
+    @Test fun forCallbacksDuringWait_notThrowOnNonNullDelegate() {
+        sessionRule.session.reload()
+        sessionRule.session.waitForPageStop()
+
+        sessionRule.session.forCallbacksDuringWait(object : Callbacks.All {
+            @AssertCalled
+            override fun onPageStop(session: GeckoSession, success: Boolean) {
+            }
+        })
+    }
+
+    @Test(expected = AssertionError::class)
     fun getCurrentCall_throwOnNoCurrentCall() {
         sessionRule.currentCall
     }
 
     @Test fun delegateUntilTestEnd() {
         var counter = 0
 
         sessionRule.delegateUntilTestEnd(object : Callbacks.ProgressDelegate {
@@ -729,16 +839,25 @@ class GeckoSessionTestRuleTest : BaseSes
                 throw IllegalStateException()
             }
         })
 
         sessionRule.session.loadTestPath(HELLO_HTML_PATH)
         sessionRule.waitForPageStop()
     }
 
+    @Test(expected = AssertionError::class)
+    @NullDelegate(GeckoSession.NavigationDelegate::class)
+    fun delegateDuringNextWait_throwOnNullDelegate() {
+        sessionRule.session.delegateDuringNextWait(object : Callbacks.NavigationDelegate {
+            override fun onLocationChange(session: GeckoSession, url: String) {
+            }
+        })
+    }
+
     @Test fun wrapSession() {
         val session = sessionRule.wrapSession(
           GeckoSession(sessionRule.session.settings))
         sessionRule.openSession(session)
         session.reload()
         session.waitForPageStop()
     }