Created Unassigned: When obfuscate a PCL Library adds a reference to mscorlib 4.0 [17]

I, am obfuscating a very simple PCL, but when I try to use it in another PCL lib it says, "The primary reference "PuenteX.Seguridad" could not be resolved because it has an indirect dependency on the framework assembly "mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" which could not be resolved in the currently targeted framework. ".NETPortable,Version=v4.0,Profile=Profile88". To resolve this problem, either remove the reference "PuenteX.Seguridad" or retarget your application to a framework version which contains "mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
When I inspect the original dll file it's do not have a reference to mscorlib 4 but after the obfucation process It's appears.

How can I solve this?

Thanks in advance.

Juan.