diff --git a/doc/geos.sgml b/doc/geos.sgml
index 204dbf7a9..b550f770e 100644
--- a/doc/geos.sgml
+++ b/doc/geos.sgml
@@ -835,8 +835,8 @@ but also removes it from drive RAM (not quite true, but it works like that). Aft
 <p>
 This function changes the logical number of the current device (in fact drives only) to the given one. It is
 usable for swapping drives. There's no check if the given <tt/newDevice/ already exist, so if you want
-to change the logical number of drive 8 to 9 and you don't have drive number 9 then GEOS will probably
-hang on disk access. Use safe, large numbers. Note that the safe IEC range is 8-31.
+to change the logical number of drive 8 to 9 and you already have a drive number 9 then GEOS will probably
+hang on disk access. Use safe, large numbers. Note that the safe IEC range is 8-30.
 
 <sect2>Disk Initialization
 <p>