Void Space Values other than 100%

Hi,

After validating a model before running it I found another error as stated below:

"The SWMM engine does not support Void Space values other than 100 percent. Alternate values are ignored."

I zoomed into it and it went to a pond and I tried to look on here and could not find anything on it. I am not sure at all what this notification is trying to say. 

Do I need to worry about it and fix something for it to go away? 

What is this notification warning me of?

Thanks!

Paola