diff --git a/js/src/builtin/TypedObject.js b/js/src/builtin/TypedObject.js
index a5f4a39d9a3e..0040d4e9bdeb 100644
--- a/js/src/builtin/TypedObject.js
+++ b/js/src/builtin/TypedObject.js
@@ -303,7 +303,8 @@ TypedObjectPointer.prototype.moveToFieldIndex = function(index) {
          "bad field descr");
   assert(TO_INT32(fieldOffset) === fieldOffset,
          "bad field offset");
-  assert(fieldOffset >= 0 && fieldOffset < DESCR_SIZE(this.descr),
+  assert(fieldOffset >= 0 &&
+         (fieldOffset + DESCR_SIZE(fieldDescr)) <= DESCR_SIZE(this.descr),
          "out of bounds field offset");
 
   this.descr = fieldDescr;
diff --git a/js/src/jit-test/tests/TypedObject/bug973563.js b/js/src/jit-test/tests/TypedObject/bug973563.js
new file mode 100644
index 000000000000..2112cc023ffb
--- /dev/null
+++ b/js/src/jit-test/tests/TypedObject/bug973563.js
@@ -0,0 +1,9 @@
+// Test that empty sized structs don't trigger any assertion failures.
+// Public domain.
+
+if (!this.hasOwnProperty("TypedObject"))
+  quit();
+
+var PointType = new TypedObject.StructType({});
+var LineType = new TypedObject.StructType({source: PointType, target: PointType});
+var fromAToB = new LineType({source: {x: 22, y: 44}, target: {x: 66, y: 88}});