SQL & PL/SQL

I am creating a view and one of the columns is this:
(b.startdate || ' To ' || b.enddate) salesDateRange
however when I was trying to handle the null value of date, I found the following problems,

case 1
(nvl(b.startdate, 'NA') || ' To ' || b.enddate) salesDateRange

when i use nvl, I am able to create the view, but when I retrieve the data, I get this error msg.
ORA-01858: a non-numeric character was found where a numeric was expected
01858. 00000 - "a non-numeric character was found where a numeric was expected"
*Cause: The input data to be converted using a date format model was
incorrect. The input data did not contain a number where a number was
required by the format model.
*Action: Fix the input data or the date format model to make sure the
elements match in number and type. Then retry the operation.

is nvl only available for numerical value?