diff --git a/tools/webidl_binder.py b/tools/webidl_binder.py
index 9f8d81dd58cb2..4fd0dac1e2bff 100644
--- a/tools/webidl_binder.py
+++ b/tools/webidl_binder.py
@@ -586,14 +586,14 @@ def render_function(class_name, func_name, sigs, return_type, non_pointer,
     if not constructor:
       if i == max_args:
         dec_args = ', '.join([type_to_cdec(raw[j]) + ' ' + args[j] for j in range(i)])
-        js_call_args = ', '.join(['%s%s' % (('(int)' if sig[j] in interfaces else '') + take_addr_if_nonpointer(raw[j]), args[j]) for j in range(i)])
+        js_call_args = ', '.join(['%s%s' % (('(ptrdiff_t)' if sig[j] in interfaces else '') + take_addr_if_nonpointer(raw[j]), args[j]) for j in range(i)])
 
         js_impl_methods.append(r'''  %s %s(%s) %s {
     %sEM_ASM_%s({
       var self = Module['getCache'](Module['%s'])[$0];
       if (!self.hasOwnProperty('%s')) throw 'a JSImplementation must implement all functions, you forgot %s::%s.';
       %sself['%s'](%s)%s;
-    }, (int)this%s);
+    }, (ptrdiff_t)this%s);
   }''' % (c_return_type, func_name, dec_args, maybe_const,
           basic_return, 'INT' if c_return_type not in C_FLOATS else 'DOUBLE',
           class_name,