get panic when deal with a ini configfile

[kokumakooli]

I'm going to add a line of configuration information to a file

original file is ini format
original content is：

[support]
abc.=def..
foo=bar
a=b

except is：

[support]
abc.=def..
foo=bar
a=b
aaaaa=bbbbb ← this is a new line

then , the key code is :

     viper.Set("support.aaaaa", "bbbbb")
     viper.WriteConfig()

when run it，i got a panic

after several checks ， I found out that it was caused by this： “abc.=def..” it with dots。 the code will be working if i delete it,
but in fact I can't delete it, and also need to insert new configuration informations in this file。
I don't know how to deal with this problem，Does anyone know how to solve this problem?
best wishes :)