Display issue when drawing self created widget

Hello,

i have created my own widget and have an display issue. Its a label like widget to draw some data( a spinlabel)
When there are no digits drawed, there are two big rectangles in the screen. Like the rectangles MS Windows displays if there is a unknown character for example.
I uploaded a screen of my problem. Display of my application
Does anyone knows if i can fix this? Its very annoying…

To create my own widget i took the lv_img.c as base

LVGL usually draws these rectangles if there is no glyph for the specific character from the font.
What font are you using? Could you share some code about your widget?