TypeError: Parameter to MergeFrom() must be instance of same class - protobuf / python 3.6.8

I'm getting this error when calling gRPC function:

TypeError: Parameter to MergeFrom() must be instance of same class: expected common_pb2.NullableString got common_pb2.NullableString.

This happens only in python 3.6.8 on RHEL 6 machine. The call passes in windows / linux with various pythons.

I have debugged this issue and found that the marked line failed (MergeFrom)

copy = field._default_constructor(self)                                                        
new_val = field_value                                                             
if isinstance(field_value, dict):                                                  
new_val = field.message_type._concrete_class(**field_value)                      
try:                                                                               
    copy.MergeFrom(new_val)    # This line fails when calling MergeFrom function
except TypeError:                                                                  
   _ReraiseTypeErrorWithFieldName(message_descriptor.name, field_name)



def MergeFrom(self, msg):                                                                           
     if not isinstance(msg, cls):                                                                      
       raise TypeError(
           'Parameter to MergeFrom() must be instance of same class: '
           'expected %s got %s.' % (_FullyQualifiedClassName(cls),
                                    _FullyQualifiedClassName(msg.__class__)))

When I print the type of new_val and copy variables, I get the same answer but if comparing them I get a false response.

print(type(new_val))                                                                                      
<class 'common_pb2.NullableString'>                                              
print(type(copy))                                                                                        
<class 'common_pb2.NullableString'>
print(type(copy) == type(new_val))
False

What can be the problem?

These are the versions of protobut and grpcio I'm using:

protobuf==3.19.4 grpcio==1.43.0

Many thanks!