diff --git a/xpcom/idl-parser/xpidl.py b/xpcom/idl-parser/xpidl.py
index 56390d8c70f..55b201fe4c5 100644
--- a/xpcom/idl-parser/xpidl.py
+++ b/xpcom/idl-parser/xpidl.py
@@ -919,6 +919,16 @@ class Method(object):
                 raise IDLError("'stringifier' attribute can only be used on methods returning DOMString",
                                self.location)
             self.iface.ops['stringifier'] = self
+        for p in self.params:
+            if p.size_is:
+                found_size_param = False
+                for size_param in self.params:
+                    if p.size_is == size_param.name:
+                        found_size_param = True
+                        if getBuiltinOrNativeTypeName(size_param.realtype) != 'unsigned long':
+                            raise IDLError("is_size parameter must have type 'unsigned long'", self.location)
+                if not found_size_param:
+                    raise IDLError("could not find is_size parameter '%s'" % p.size_is, self.location)
 
     def isScriptable(self):
         if not self.iface.attributes.scriptable: return False