diff --git a/doc/src/sgml/pgfreespacemap.sgml b/doc/src/sgml/pgfreespacemap.sgml
index 0122d278e39..7647e6512ea 100644
--- a/doc/src/sgml/pgfreespacemap.sgml
+++ b/doc/src/sgml/pgfreespacemap.sgml
@@ -66,7 +66,7 @@
   <para>
    For indexes, what is tracked is entirely-unused pages, rather than free
    space within pages.  Therefore, the values are not meaningful, just
-   whether a page is full or empty.
+   whether a page is in-use or empty.
   </para>
 
   <note>
diff --git a/src/backend/storage/freespace/indexfsm.c b/src/backend/storage/freespace/indexfsm.c
index d975c3364b4..4788e7e7183 100644
--- a/src/backend/storage/freespace/indexfsm.c
+++ b/src/backend/storage/freespace/indexfsm.c
@@ -16,7 +16,7 @@
  *	This is similar to the FSM used for heap, in freespace.c, but instead
  *	of tracking the amount of free space on pages, we only track whether
  *	pages are completely free or in-use. We use the same FSM implementation
- *	as for heaps, using BLCKSZ - 1 to denote used pages, and 0 for unused.
+ *	as for heaps, using 0 to denote used pages, and (BLCKSZ - 1) for unused.
  *
  *-------------------------------------------------------------------------
  */