Return "no such frame" error instead "invalid argument"

index.bs

@@ -1143,7 +1143,7 @@ To <dfn>get related navigables</dfn> given an [=script/settings object=]
 
 <div algorithm>
 
-To <dfn>get navigables by ids</dfn> given a [=/list=] of context ids |navigable ids|:
+To <dfn>get navigables by ids</dfn> given a [=/list=] of context ids |navigable ids| and |validate navigables|:
 
 1. Let |result| be an empty [=/set=].
 
@@ -1152,7 +1152,10 @@ To <dfn>get navigables by ids</dfn> given a [=/list=] of context ids |navigable
    1. Let |navigable| be the [=/navigable=] with id |navigable id|
       if such [=/navigable=] exists, and null otherwise.
 
-   1. [=set/Append=] |navigable| to |result| if |navigable| is not null.
+   1. If |navigable| is not null, [=set/append=] |navigable| to |result|.
+
+   1. Otherwise, if |validate navigables| is true return [=error=] with
+      [=error code=] [=no such frame=].
 
 1. Return |result|.
 
@@ -1982,11 +1985,9 @@ 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. Let |validate navigables| be true.
 
-      1. Return [=error=] with [=error code=] [=invalid argument=].
+   1. Let |navigables| be [=get navigables by ids=] with |input context ids| and |validate navigables|.
 
    1. Set |subscription navigables| be [=get top-level traversables=] with |navigables|.
 
@@ -2102,11 +2103,9 @@ 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. Let |validate navigables| be true.
 
-         1. Return [=error=] with [=error code=] [=invalid argument=].
+      1. Let |navigables| be [=get navigables by ids=] with |input context ids| and |validate navigables|.
 
       1. Set |top-level traversables| be [=get top-level traversables=] with |navigables|.