Why would this query return an error that mentions an entirely different database being offline?

trying list of fragmented indexes based on database name.  however, 

select	count(*) tablefragover80pctcount sys.dm_db_index_physical_stats (null, null, null, null, null )as ts inner join sys.tables t on t.[object_id] = ts.[object_id] inner join sys.indexes on i.[object_id] = ts.[object_id] inner join sys.databases db on ts.database_id = db.database_id db.name = 'my db name' , ts.avg_fragmentation_in_percent  > 80.00

the error back 

msg 942, level 14, state 4, line 1
database 'very different db' cannot opened because offline.

i know there database on server offline, 2 or 3, not 1 im trying information about.  what cause above query have trouble getting index info caused entirely different database being offline?

from sys.dm_db_index_physical_stats (null, null, null, null, null )as ts

see msdn sys.dm_db_index_physical_stats ;  first parameter database id; if pass null here, databases scanned, offline one.

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