diff --git a/storage/storage_dest.go b/storage/storage_dest.go
index f5fb3603f7..5f54459e9f 100644
--- a/storage/storage_dest.go
+++ b/storage/storage_dest.go
@@ -88,7 +88,8 @@ type storageImageDestinationLockProtected struct {
 	// Layer identification: For a layer, at least one of indexToTOCDigest and blobDiffIDs must be available before commitLayer is called.
 	blobDiffIDs      map[digest.Digest]digest.Digest // Mapping from layer blobsums to their corresponding DiffIDs
 	indexToTOCDigest map[int]digest.Digest           // Mapping from layer index to a TOC Digest, IFF the layer was created/found/reused by TOC digest
-
+	// Layer data: For a layer, at least one of (diffOutputs, blobAdditionalLayer, filenames, FIXME layer)
+	// must be available before commitLayer is called:
 	fileSizes             map[digest.Digest]int64                     // Mapping from layer blobsums to their sizes
 	filenames             map[digest.Digest]string                    // Mapping from layer blobsums to names of files we used to hold them
 	currentIndex          int                                         // The index of the layer to be committed (i.e., lower indices have already been committed)