diff --git a/news/15.bugfix b/news/15.bugfix
new file mode 100644
index 0000000..d28742e
--- /dev/null
+++ b/news/15.bugfix
@@ -0,0 +1,3 @@
+Fix error when a class is extended to add others aggregated classes as base
+classes. Before this fix, the builder used the list of all base classes
+the final class should have as base classes for every class into the hierarchy.
diff --git a/src/extendable/main.py b/src/extendable/main.py
index 0638288..2360bbc 100644
--- a/src/extendable/main.py
+++ b/src/extendable/main.py
@@ -24,6 +24,7 @@
 class ExtendableClassDef:
     name: str
     base_names: List[str]
+    original_base_names: List[str]
     namespace: Dict[str, Any]
     original_name: str
     others_bases: List[Any]
@@ -44,7 +45,8 @@ def __init__(
         self.name = namespace["__xreg_name__"]
         self.original_name = original_name
         self.others_bases = bases
-        self.base_names = namespace["__xreg_base_names__"] or []
+        self.base_names = namespace["__xreg_base_names__"].copy() or []
+        self.original_base_names = self.base_names.copy()
         self.hierarchy = [self]
         self.metaclass = metaclass
         self.kwargs = kwargs
diff --git a/src/extendable/registry.py b/src/extendable/registry.py
index afd1e46..5092048 100644
--- a/src/extendable/registry.py
+++ b/src/extendable/registry.py
@@ -121,13 +121,18 @@ def build_extendable_class(
             # determine all the classes the component should inherit from
             bases = LastOrderedSet[main.ExtendableMeta]()
             for base_name in cls_def.base_names:
+                # the base_names contains all the bases for the final aggregated
+                # class. Here we check that all the base required to build the
+                # current hierarchy are already build.
                 if base_name not in self:
                     if idx != 0 or base_name != cls_def.name:
                         raise TypeError(
                             f"Pydnatic class '{name}' extends an non-existing "
                             f"extendable class '{base_name}'."
                         )
-                else:
+                elif base_name in cls_def.original_base_names:
+                    # The bases to inherit for the current class are the one
+                    # defined into the original class definition.
                     parent_class = self[base_name]
                     bases.add(parent_class)
             for other_base in class_def.others_bases:
diff --git a/tests/test_simple.py b/tests/test_simple.py
index 7691550..6ae6e3a 100644
--- a/tests/test_simple.py
+++ b/tests/test_simple.py
@@ -258,6 +258,35 @@ class MyExt(metaclass=MyMeta):
     assert MyMeta._is_extendable(MyExt)
 
 
+def test_mixin_inheritance(test_registry):
+    class BaseMixin(metaclass=ExtendableMeta):
+        def test(self):
+            return "base"
+
+    class MixinA(BaseMixin):
+        def test_a(self):
+            return "A"
+
+    class MixinB(BaseMixin):
+        def test_b(self):
+            return "B"
+
+    class ExtendedB(MixinB, extends=True):
+        def test_b(self):
+            res = super().test_b()
+            return res + " extended"
+
+    class Mixin(MixinA, MixinB, extends=MixinB):
+        pass
+
+    test_registry.init_registry()
+
+    obj = Mixin()
+    assert obj.test() == "base"
+    assert obj.test_a() == "A"
+    assert obj.test_b() == "B extended"
+
+
 def test_issubclass_multi_level(test_registry):
     class A(metaclass=ExtendableMeta):
         pass