diff --git a/doc/src/sgml/ref/pg_basebackup.sgml b/doc/src/sgml/ref/pg_basebackup.sgml
index 2b7443b93fd..40d92960edb 100644
--- a/doc/src/sgml/ref/pg_basebackup.sgml
+++ b/doc/src/sgml/ref/pg_basebackup.sgml
@@ -155,7 +155,8 @@ PostgreSQL documentation
             the target directory. If the cluster contains additional
             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 server.
+            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.
@@ -291,7 +292,12 @@ 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 ba2fbdcf02a..126ea6521ee 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -11517,7 +11517,7 @@ 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;
 	}