In 13.2 "Arrow Function Definitions",
under "Runtime Semantics: Binding Initialisation",
in the first rule,
step 1 says:
"Return the result of performing Indexed Binding Initialisation
for BindingIdentifier using value, lenValue, 0, and environment
as the arguments."
and in the fifth rule,
step 3 says:
"Return the result of performing Indexed Binding Initialisation
for FunctionRestParameter using value, lenValue, restIndex,
and environment as the arguments."
In each case, 4 arguments are supplied, but Indexed Binding Initialisation only accepts 3, so delete "lenValue," (which isn't defined anyway).
fixed in editor's draft