USE Database Statement

I'm having a problem with the following script:
use master
declare database_cursor CURSOR for
SELECT name from sysdatabases
OPEN database_cursor
DECLARE @databasename varchar (30)
FETCH next from database_cursor into @databasename
while (@@fetch_status <> -1)

BEGIN
use @databasename
exec sp_spaceused
FETCH next from database_cursor into @databasename
END

The substitution of database name in the USE statement does not work correctly. It substitutes in the length, '30', from the declare variable statement. I've had one suggestion to try declaring the variable as DECLARE @DATABASENAME SYSNAME but that has not worked. Any ideas?