diff --git a/spec.bs b/spec.bs
index 10a901b..ee8436d 100644
--- a/spec.bs
+++ b/spec.bs
@@ -666,16 +666,20 @@ For now, see [https://github.com/wicg/observable#operators](https://github.com/w
 
        1. Let |controller| be a [=new=] {{AbortController}}.
 
+       1. Let |idx| be an {{unsigned long long}}, initially 0.
+
        1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:
 
           : [=internal observer/next steps=]
-          :: 1. [=Invoke=] |mapper| with the passed in <var ignore>value</var>, and let
+          :: 1. [=Invoke=] |mapper| with the passed in <var ignore>value</var>, and |idx|, and let
                 |mappedValue| be the returned value.
 
                 If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>,
                 then run |subscriber|'s {{Subscriber/error()}} method, given |E|,
                 [=AbortController/signal abort=] |controller|, and abort these steps.
 
+             1. Increment |idx|.
+
              1. Run |subscriber|'s {{Subscriber/next()}} method, given |mappedValue|.
 
           : [=internal observer/error steps=]