--- a/testing/marionette/session.js
+++ b/testing/marionette/session.js
@@ -48,33 +48,31 @@ session.Timeouts = class {
     };
   }
 
   static fromJSON (json) {
     assert.object(json);
     let t = new session.Timeouts();
 
     for (let [typ, ms] of Object.entries(json)) {
-      assert.positiveInteger(ms);
-
       switch (typ) {
         case "implicit":
-          t.implicit = ms;
+          t.implicit = assert.positiveInteger(ms);
           break;
 
         case "script":
-          t.script = ms;
+          t.script = assert.positiveInteger(ms);
           break;
 
         case "pageLoad":
-          t.pageLoad = ms;
+          t.pageLoad = assert.positiveInteger(ms);
           break;
 
         default:
-          throw new InvalidArgumentError();
+          throw new InvalidArgumentError("Unrecognised timeout: " + typ);
       }
     }
 
     return t;
   }
 };
 
 /** Enum of page loading strategies. */

--- a/testing/marionette/test_session.js
+++ b/testing/marionette/test_session.js
@@ -43,16 +43,53 @@ add_test(function test_Timeouts_fromJSON
   let ts = session.Timeouts.fromJSON(json);
   equal(ts.implicit, json.implicit);
   equal(ts.pageLoad, json.pageLoad);
   equal(ts.script, json.script);
 
   run_next_test();
 });
 
+add_test(function test_Timeouts_fromJSON_unrecognised_field() {
+  let json = {
+    sessionId: "foobar",
+    script: 42,
+  };
+  try {
+    session.Timeouts.fromJSON(json);
+  } catch (e) {
+    equal(e.name, InvalidArgumentError.name);
+    equal(e.message, "Unrecognised timeout: sessionId");
+  }
+
+  run_next_test();
+});
+
+add_test(function test_Timeouts_fromJSON_invalid_type() {
+  try {
+    session.Timeouts.fromJSON({script: "foobar"});
+  } catch (e) {
+    equal(e.name, InvalidArgumentError.name);
+    equal(e.message, "Expected [object String] \"foobar\" to be an integer");
+  }
+
+  run_next_test();
+});
+
+add_test(function test_Timeouts_fromJSON_bounds() {
+  try {
+    session.Timeouts.fromJSON({script: -42});
+  } catch (e) {
+    equal(e.name, InvalidArgumentError.name);
+    equal(e.message, "Expected [object Number] -42 to be >= 0");
+  }
+
+  run_next_test();
+});
+
 add_test(function test_PageLoadStrategy() {
   equal(session.PageLoadStrategy.None, "none");
   equal(session.PageLoadStrategy.Eager, "eager");
   equal(session.PageLoadStrategy.Normal, "normal");
 
   run_next_test();
 });