Dynamic Data Source Expression

When i attempt to create a dynamic data source using an expression,
having tested the report with a constant data source string, the report
compilation fails with this error:
[rsUnexpectedCompilerError] An unexpected error occurred while
compiling expressions. Native compiler return value: '[BC32017]
Comma, ')', or a valid expression continuation expected.'
I am using Visual Studio 2005.
Simply changing the data source string from:
Data source = xxx;intial catalog=yyy
to:
=Data source = xxx;intial catalog=yyy
causes this error - i.e. I am not even referencing any parameters.
Has anyone encountered this before? Any resolutions?Amendment:
=Data source = xxx;intial catalog=yyy
should read:
="Data source = xxx;intial catalog=yyy"