Can't add zero to text block

To convert a text input’s text from a string to a number, you can add zero using a math block. But when I try to do the same thing with a label’s text, it won’t allow the block to connect:

Shouldn’t I be able to connect a substring block or even a basic text block to the math block?

I realize I can do this as a workaround but it shouldn’t be necessary:

Maybe colon : are problem? Math block should just have decimal and thousands separators.

The colons are part of the variable name, not part of the variable value. That won’t have an effect on other blocks. The values I’m using are hour values from a clock such as “05” or “12”.

After get substring those value is 05. Before that it have colon and block is red color, so type text. Try write 05 as text and then add it into math block. Just for test so we can see what happend.