diff --git a/index.bs b/index.bs
index 2d7c6417..c3574247 100644
--- a/index.bs
+++ b/index.bs
@@ -1172,6 +1172,22 @@ To <dfn>get top-level traversables</dfn> given a [=/list=] of [=/navigables=] |n
 
 </div>
 
+<div algorithm>
+
+To <dfn>get valid navigables by ids</dfn> given a [=/list=] of context ids |navigable ids|:
+
+1. Let |result| be an empty [=/set=].
+
+1. For each |navigable id| in |navigable ids|:
+
+   1. Let |navigable| be the result of [=trying=] to [=get a navigable=] with |navigable id|.
+
+   1. [=set/Append=] |navigable| to |result|.
+
+1. Return [=success=] with data |result|.
+
+</div>
+
 <div algorithm> To <dfn export>emit an event</dfn> given |session|, and |body|:
 
 1. [=Assert=]: |body| has [=map/size=] 2 and [=map/contains=] "<code>method</code>"
@@ -1982,11 +1998,7 @@ The [=remote end steps=] with |session| and |command parameters| are:
 
 1. If |input context ids| is not empty:
 
-   1. Let |navigables| be [=get navigables by ids=] with |input context ids|.
-
-   1. If [=list/size=] of |navigables| does not equal [=list/size=] of |input context ids|:
-
-      1. Return [=error=] with [=error code=] [=invalid argument=].
+   1. Let |navigables| be the result of [=trying=] to [=get valid navigables by ids=] with |input context ids|.
 
    1. Set |subscription navigables| be [=get top-level traversables=] with |navigables|.
 
@@ -2102,11 +2114,7 @@ The [=remote end steps=] with |session| and |command parameters| are:
 
    1. If |input context ids| is not empty:
 
-      1. Let |navigables| be [=get navigables by ids=] with |input context ids|.
-
-      1. If [=list/size=] of |navigables| does not equal [=list/size=] of |input context ids|:
-
-         1. Return [=error=] with [=error code=] [=invalid argument=].
+      1. Let |navigables| be the result of [=trying=] to [=get valid navigables by ids=] with |input context ids|.
 
       1. Set |top-level traversables| be [=get top-level traversables=] with |navigables|.