diff --git a/sympde/topology/basic.py b/sympde/topology/basic.py
index bce622f3..bc04395a 100644
--- a/sympde/topology/basic.py
+++ b/sympde/topology/basic.py
@@ -136,11 +136,11 @@ def __new__(cls, *args):
         # remove duplicates and sort domains by their string representation
         args = sorted(set(args), key=str)
 
-        # a. If the required Union contains no domains, return None;
+        # a. If the required Union contains no domains, return an empty tuple;
         # b. If it contains a single domain, return the domain itself;
         # c. If it contains multiple domains, create a Union object.
         if not args:
-            obj = None
+            obj = ()
         elif len(args) == 1:
             obj = args[0]
         else:
diff --git a/sympde/topology/domain.py b/sympde/topology/domain.py
index 12627ac8..fc1ea476 100644
--- a/sympde/topology/domain.py
+++ b/sympde/topology/domain.py
@@ -75,12 +75,12 @@ def __new__(cls, name : str, *,
         # ...
 
         # ...
-        if ( ( interiors is None ) and ( connectivity is None ) and ( dim is None) ):
+        if ( ( interiors in [None, ()] ) and ( connectivity in [None, ()] ) and ( dim is None) ):
             raise ValueError('> either interiors or connectivity must be given')
         # ...
 
         # ...
-        if not( interiors is None ):
+        if not( interiors in [None, ()] ):
             if not isinstance( interiors, (*iterable_types, InteriorDomain)):
                 raise TypeError('> Expecting an iterable or a InteriorDomain')
 
@@ -103,7 +103,7 @@ def __new__(cls, name : str, *,
             interiors = Tuple(*interiors)
         # ...
 
-        if not( boundaries is None ):
+        if not( boundaries in [None, ()] ):
             if not isinstance( boundaries, (*iterable_types, Boundary)):
                 raise TypeError('> Expecting an iterable or a Boundary')
 
@@ -119,7 +119,7 @@ def __new__(cls, name : str, *,
 
         boundaries = Tuple(*boundaries)
 
-        if not( connectivity is None ):
+        if not( connectivity in [None, ()] ):
             if not isinstance( connectivity, Connectivity ):
                 raise TypeError('> Expecting a Connectivity')
 
@@ -128,7 +128,7 @@ def __new__(cls, name : str, *,
             connectivity = Connectivity()
 
         # ...
-        if interiors is None and dim:
+        if interiors in [None, ()] and dim:
             interiors = [InteriorDomain(name, dim=dim)]
 
         if len(interiors) == 0 and dim is None:
@@ -228,7 +228,7 @@ def interfaces(self) -> TypeUnion[Union, Interface, None]:
     @property
     def corners(self):
         corners = getattr(self,'_corners', None)
-        if corners is None:
+        if corners in [None, ()]:
             corners = self.get_shared_corners()
         self._corners = corners
         return corners
@@ -540,7 +540,7 @@ def join(cls, patches, connectivity, name):
 
         # ... boundary
         boundaries = Union(*[b for p in patches for b in p.boundary]).complement(Union(*boundaries))
-        if boundaries is None:
+        if boundaries in [None, ()]:
             boundaries = ()
         else :
             boundaries = boundaries.as_tuple()