Query Conversion Issue

i am trying to execute the following query but I am getting an error because the data types are different:

SELECT DISTINCT CurrentlyOnline.USERID, CurrentlyOnline.IPADDRESS, aspnet_Users.UserName
FROM CurrentlyOnline INNER JOIN
aspnet_Users ON CurrentlyOnline.USERID = aspnet_Users.UserId


Msg 8169, Level 16, State 2, Line 1Conversion failed when converting from a character string to uniqueidentifier.

In the CurrentlyOnline table the USERID field is defined as varchar (150)
In the aspnet_Users table the UserId field is defined as uniqueidentifier

Any ideas on how do make the query work?