diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index f1ae736ef3f..82238f70080 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -14400,7 +14400,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
        <entry><type>bigint</type></entry>
        <entry>
         Disk space used by the specified fork (<literal>'main'</literal>,
-        <literal>'fsm'</literal> or <literal>'vm'</>)
+        <literal>'fsm'</literal>, <literal>'vm'</>, or <literal>'init'</>)
         of the specified table or index
        </entry>
       </row>
@@ -14521,6 +14521,12 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
        (see <xref linkend="storage-vm">) associated with the relation.
       </para>
      </listitem>
+     <listitem>
+      <para>
+       <literal>'init'</literal> returns the size of the initialization fork,
+       if any, (see <xref linkend="storage-init">) associated with the relation.
+      </para>
+     </listitem>
     </itemizedlist>
    </para>
 
diff --git a/src/backend/catalog/catalog.c b/src/backend/catalog/catalog.c
index 03799b5ba6a..79c1a1d01d1 100644
--- a/src/backend/catalog/catalog.c
+++ b/src/backend/catalog/catalog.c
@@ -74,7 +74,8 @@ forkname_to_number(char *forkName)
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid fork name"),
-			 errhint("Valid fork names are \"main\", \"fsm\", and \"vm\".")));
+			 errhint("Valid fork names are \"main\", \"fsm\", "
+					 "\"vm\", and \"init\".")));
 	return InvalidForkNumber;	/* keep compiler quiet */
 }