diff --git a/pstoremem/addr_book.go b/pstoremem/addr_book.go
index e72edd6..f0f8a2c 100644
--- a/pstoremem/addr_book.go
+++ b/pstoremem/addr_book.go
@@ -45,6 +45,7 @@ func NewAddrBook() pstore.AddrBook {
 	}
 }
 
+// gc garbage collects the in-memory address book. The caller *must* hold the addrmu lock.
 func (mab *memoryAddrBook) gc() {
 	now := time.Now()
 	if !now.After(mab.nextGC) {