diff --git a/pina/_src/domain/base_domain.py b/pina/_src/domain/base_domain.py
index 3316fabfd..d3cea3848 100644
--- a/pina/_src/domain/base_domain.py
+++ b/pina/_src/domain/base_domain.py
@@ -103,8 +103,17 @@ def update(self, domain):
                 f"with domain of type {type(domain)}."
             )
 
-        # Update fixed and ranged variables
+        # Create a deepcopy of the current domain
         updated = deepcopy(self)
+
+        # Remove keys that change category
+        for key in domain.fixed:
+            updated.range.pop(key, None)
+
+        for key in domain.range:
+            updated.fixed.pop(key, None)
+
+        # Update fixed and ranged variables
         updated.fixed.update(domain.fixed)
         updated.range.update(domain.range)
 
diff --git a/tests/test_domain/test_cartesian_domain.py b/tests/test_domain/test_cartesian_domain.py
index db9297ced..e5b80a5ad 100644
--- a/tests/test_domain/test_cartesian_domain.py
+++ b/tests/test_domain/test_cartesian_domain.py
@@ -76,8 +76,8 @@ def test_update(dict):
 
     # Define the domains
     domain_1 = CartesianDomain(dict)
-    domain_2 = CartesianDomain({"new_var": [0, 1]})
-    domain_3 = CartesianDomain(dict | {"new_var": [0, 1]})
+    domain_2 = CartesianDomain({"new_var": [0, 1], "x": 1})
+    domain_3 = CartesianDomain(dict | {"new_var": [0, 1], "x": 1})
 
     # Update domain_1 with domain_2
     updated_domain = domain_1.update(domain_2)
diff --git a/tests/test_domain/test_ellipsoid_domain.py b/tests/test_domain/test_ellipsoid_domain.py
index ced0f9dd0..a39d3eca2 100644
--- a/tests/test_domain/test_ellipsoid_domain.py
+++ b/tests/test_domain/test_ellipsoid_domain.py
@@ -91,10 +91,12 @@ def test_update(dict, sample_surface):
         ellipsoid_dict=dict, sample_surface=sample_surface
     )
     domain_2 = EllipsoidDomain(
-        ellipsoid_dict={"new_var": [0, 1]}, sample_surface=sample_surface
+        ellipsoid_dict={"new_var": [0, 1], "x": 1},
+        sample_surface=sample_surface
     )
     domain_3 = EllipsoidDomain(
-        ellipsoid_dict=dict | {"new_var": [0, 1]}, sample_surface=sample_surface
+        ellipsoid_dict=dict | {"new_var": [0, 1], "x": 1},
+        sample_surface=sample_surface
     )
 
     # Update domain_1 with domain_2