Internal INTEGER error in track hash

[brodieG]

Can't reproduce this consistently. Seems like it only happens the first time the code is run.

> vetr:::track_hash(letters[1:5], 2L)
Error in vetr:::track_hash(letters[1:5], 2L) : 
  INTEGER() can only be applied to a 'integer', not a 'NULL'

| Value mismatch: 

< .ref           > .new         
@@ 1 @@          @@ 1 @@        
< [1] 1 1 4 1 8  > NULL         

| Conditions mismatch: 

< .REF$conditions                                                         
> .NEW$conditions                                                         
@@ 1 / 1,3 @@                                                             
< Empty condition list                                                    
> Condition list with 1 condition:                                        
> 1. Error in vetr:::track_hash(letters[1:5], 2L) : INTEGER() can only be 
>    applied to a 'integer', not a 'NULL'