Problem with filtering data

Hi!

I'm having trouble with filtering data from my table and don't know how to resolve it properly.

I created a tabular form my table, a few text fields that are used as input for different columns of the filter and a filter button that submits the page.

The problem appeared when I try to filter a column that has null values in the table and is editable...that's as far as I have concluded.
So when I press the button I get zero rows even though there should be some.

This is the submitted query:

select

"MSR_PRD_ID",

"SRC_STM_ID",

"ID",

"ISIN",

...

from "#OWNER#"."IZV_SLOG_DET"

WHERE

lower(MSR_PRD_ID) = lower(nvl(:P7_X_MSR_PRD_ID,MSR_PRD_ID)) and

lower(SRC_STM_ID) = lower(nvl(:P7_X_SRC_STM_ID,SRC_STM_ID)) and

lower(ISIN) = lower(nvl(:P7_X_ISIN,ISIN)) and

...

The nvl function is used so that if the user leaves the field empty it only filters by the fields that have input.

Does anyone know how I could circumvent this?

Regards,

Ivan