diff --git a/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py b/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py
index f50fdc568438..5c9c92d0376b 100644
--- a/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py
@@ -223,9 +223,14 @@ class TestCapabilityMatching(MarionetteTestCase):
             self.marionette.start_session({"desiredCapabilities": {"pageLoadStrategy": strategy}})
             self.assertEqual(self.marionette.session_capabilities["pageLoadStrategy"], strategy)
 
+        # A null value should be treatend as "normal"
+        self.delete_session()
+        self.marionette.start_session({"desiredCapabilities": {"pageLoadStrategy": None}})
+        self.assertEqual(self.marionette.session_capabilities["pageLoadStrategy"], "normal")
+
         for value in ["", "EAGER", True, 42, {}, []]:
             print("invalid strategy {}".format(value))
-            with self.assertRaises(SessionNotCreatedException):
+            with self.assertRaisesRegexp(SessionNotCreatedException, "InvalidArgumentError"):
                 self.marionette.start_session({"desiredCapabilities": {"pageLoadStrategy": value}})
 
     def test_proxy_default(self):
diff --git a/testing/marionette/session.js b/testing/marionette/session.js
index f353b6407b57..c47423e52def 100644
--- a/testing/marionette/session.js
+++ b/testing/marionette/session.js
@@ -245,7 +245,7 @@ session.Capabilities = class extends Map {
       throw new TypeError();
     }
 
-    return super.set(key, value);  
+    return super.set(key, value);
   }
 
   toString() { return "[object session.Capabilities]"; }
@@ -357,11 +357,18 @@ session.Capabilities = class extends Map {
           break;
 
         case "pageLoadStrategy":
-          if (Object.values(session.PageLoadStrategy).includes(v)) {
-            matched.set("pageLoadStrategy", v);
+          if (v === null) {
+            matched.set("pageLoadStrategy", session.PageLoadStrategy.Normal);
           } else {
-            throw new TypeError("Unknown page load strategy: " + v);
+            assert.string(v);
+
+            if (Object.values(session.PageLoadStrategy).includes(v)) {
+              matched.set("pageLoadStrategy", v);
+            } else {
+              throw new InvalidArgumentError("Unknown page load strategy: " + v);
+            }
           }
+
           break;
 
         case "proxy":