Adaptive threshold return empty characters

Question

When I adaptive threshold an image, in some cases it returns empty characters. I am looking for a solution, maybe some preprocessing to prevent this. Here are some examples. (photo from google)

and the result:

I want something like the small size numbers (0123... - which are solid). It also happens when we have light characters on dark background! Thank you.

edit: here is a number on dark bg. I used a large kernel size but it is not resolved.