Double types

I have an application that writes an Excel Spreadsheet to postgres. For the values that go in number fields, I check the Excel values for dbnull and set the parameters to 0, like this: cmd.Parameters(9).Value = 0. Npgsql throws an error “format specifier was invalid” If I do this: cmd.Parameters(9).Value = convert.todouble(0) then it works fine.

Visual Basic 2010, Postgres 9.1, npgsql 2.0.12.0

Could somebody please explain this behavior?