How to add curves without getting cutouts

[brittany13]

Hey everyone! Just trying to figure out how to add curves together in a way that it still looks like the first picture. Is there a way to do this? Every time I try to combine the shapes, I end up with the second picture. Is it because the lines were drawn with the brush tool? Sorry if this is a silly question, I'm still learning 

Thanks!!

[gdenby]

Usually when something like this happens its because the shapes were unclosed curves.When a boolean operation like add is used on 2 or more, the operation closes any open curve, adding a stroke to the new connection between the old first and last nodes.

[PixelPest]

You might want to "Expand stroke" before Boolean Add curves+shapes:

Cheers

[brittany13]

Oh great! Thank you both so much, this helped a lot!