A column of all records becomes null, if set null as argument to replace function

Issue

• When invoking query with replace function with null value to vdb, a column of all records becomes null.
  In the following example, replace function is replacing 'col2' to null value regardless whether the value is 'a' or not.

select col1, replace(col2, 'a', null) from vdb_table

Results
1:  0, null
2:  1, null
3:  2, null
4:  3, null

Results (Expected)
1:  0, null
2:  1, b
3:  2, c
4:  3, d