export i fails to work in bash

I ran the following commands:
```
# In parent shell
export a=1
export i=2
bash # open child shell
# In child shell
echo $a # 1
echo $i # (empty)
exit
```

This bug happens when exporting the "i" variable from parent shell to child shell.
I have not tested what other variable names would trigger this bug.

* My OS: Ubuntu 20.04 LTS
* Bash version (running `bash --version`):
```
GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
```

I have also tested in the `sh` shell and did not see the problem:
```
# In parent shell
export a=1
export i=2
sh # open child shell
# In child shell
echo $a # 1
echo $i # 2
exit
```