diff --git a/reframe/core/variables.py b/reframe/core/variables.py
index ef1fce3603..4a110358b5 100644
--- a/reframe/core/variables.py
+++ b/reframe/core/variables.py
@@ -17,6 +17,9 @@ class _UndefinedType:
     '''Custom type to flag a variable as undefined.'''
     __slots__ = ()
 
+    def __deepcopy__(self, memo):
+        return self
+
 
 _Undefined = _UndefinedType()
 
@@ -116,7 +119,7 @@ def join(self, other, cls):
                     f'parent classes of class {cls.__qualname__!r}'
                 )
 
-            self.vars[key] = var
+            self.vars[key] = copy.deepcopy(var)
 
         # Carry over the set of injected variables
         self._injected_vars.update(other._injected_vars)
diff --git a/unittests/test_variables.py b/unittests/test_variables.py
index 0c982bf33c..1f9a4d44e5 100644
--- a/unittests/test_variables.py
+++ b/unittests/test_variables.py
@@ -178,8 +178,13 @@ def __init__(self):
     class Bar(Base):
         pass
 
+    class Baz(Base):
+        my_var = []
+
+    assert Base().my_var == [1, 2]
     assert Foo().my_var == [1, 2, 3]
     assert Bar().my_var == [1, 2]
+    assert Baz().my_var == []
 
 
 def test_variable_access():