I might not have chased this bug fully[1] but what I've observed is the following:

When checking dbhashes across replica set nodes, a collection that's on the primary, but missing on a secondary will proceed to compare collStats for the missing collection. A collstats for a missing collection:

test> db.server848.stats()
{
	"ns" : "test.server848",
	"size" : 0,
	"count" : 0,
	"storageSize" : 0,
	"totalSize" : 0,
	"nindexes" : 0,
	"totalIndexSize" : 0,
	"indexSizes" : {
		
	},
	"scaleFactor" : 1,
	"ok" : 1
}

First observation of note is that calling collStats on a missing collection returns ok: 1 avoiding the early return.

Second, the indexBuilds field is (javascript) undefined.

If the primary's indexBuilds is undefined (leftArr), the code is robust.

If the secondary's indexBuilds is undefined (rightArr), a type error is hit.

[1] It's not obvious to me whether something changed in how collStats responds, or if this was always a failure waiting to happen.