diff --git a/doc/src/sgml/ref/pg_basebackup.sgml b/doc/src/sgml/ref/pg_basebackup.sgml
index 14cc88a8522..8ce103301f2 100644
--- a/doc/src/sgml/ref/pg_basebackup.sgml
+++ b/doc/src/sgml/ref/pg_basebackup.sgml
@@ -161,6 +161,7 @@ PostgreSQL documentation
             tablespaces, the main data directory will be placed in the
             target directory, but all other tablespaces will be placed
             in the same absolute path as they have on the source server.
+            (See <option>--tablespace-mapping</option> to change that.)
            </para>
            <para>
             This is the default format.
@@ -241,7 +242,12 @@ PostgreSQL documentation
         the main data directory are updated to point to the new location.  So
         the new data directory is ready to be used for a new server instance
         with all tablespaces in the updated locations.
-        </para>
+       </para>
+
+       <para>
+        Currently, this option only works with plain output format; it is
+        ignored if tar format is selected.
+       </para>
       </listitem>
      </varlistentry>
 
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index f4d1ce5deae..9be0cb236d3 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -11959,7 +11959,7 @@ read_tablespace_map(List **tablespaces)
 		}
 		else if ((ch == '\n' || ch == '\r') && prev_ch == '\\')
 			str[i - 1] = ch;
-		else
+		else if (i < sizeof(str) - 1)
 			str[i++] = ch;
 		prev_ch = ch;
 	}