How to find a namespace of base class via Roslyn

I'am writing source generator that makes a copy (with some little changes) of simple DTO classes. I have copied all contained properties and all works fine. But some of my classes inherited from another like this:

public class MyClass : MyBaseClass
{
    public string MyProperty { get; set; }
}

I can't just copy inheritance declaration, because my copied classes locaded in different namespace. So I need to add using to namespace of MyBaseClass, so I need to find than namespace. I've already done this for my property types using semantic model, like this:

Compilation
   .GetSemanticModel(propertyNode.Type.SyntaxTree)
   .GetSymbolInfo(propertyNode.Type)
   .Symbol
   .ContainingNamespace

But I have unexpected issue with gettting namespace from base type. What i've tried for now:

// list of all base types
var typeList = classNode.BaseList;                  
var semanticModel = Compilation.GetSemanticModel(typeList.SyntaxTree);

var symbol = semanticModel.GetSymbolInfo(typeList); // symbol.Symbol is null
var type = semanticModel.GetTypeInfo(typeList);     // type.Type is null 

// list of all base types
var firstType = classNode.BaseList.Types[0]; 
var semanticModel = Compilation.GetSemanticModel(firstType.SyntaxTree);

var symbol = semanticModel.GetSymbolInfo(firstType); // symbol.Symbol is null
var type = semanticModel.GetTypeInfo(firstType);     // type.Type is null 

// list of all base types
var typeList = classNode.BaseList;
var semanticModel = Compilation.GetSemanticModel(typeList.SyntaxTree);

var symbol = semanticModel.GetSymbolInfo(typeList.Types[0]); // symbol.Symbol is null
var type = semanticModel.GetTypeInfo(typeList.Types[0]);     // type.Type is null 

I dont know what went wrong. I've tried to call different methods with different parameters, but did not get proper result. May be there is another way to do this?