diff --git a/src/Build/Province.php b/src/Build/Province.php
index 9385455..a1847a0 100644
--- a/src/Build/Province.php
+++ b/src/Build/Province.php
@@ -58,18 +58,6 @@ public function checkMunicipalityData(stdClass $data): void
         }
     }
 
-    public function checkFlatEntry(FlatMunicipality $flatMunicipality): void
-    {
-        if ($this->id !== $flatMunicipality->utsID
-            || $this->oldID !== $flatMunicipality->utsOldID
-            || $this->vehicleCode !== $flatMunicipality->utsVehicleCode
-            || $this->type !== $flatMunicipality->utsType
-            || $this->name !== $flatMunicipality->utsName
-        ) {
-            throw new RuntimeException('Incompatible FlatMunicipality for ' . __CLASS__);
-        }
-    }
-
     public function serialize(Collator $collator): array
     {
         $result = [
diff --git a/test/tests/SingletonsTest.php b/test/tests/SingletonsTest.php
index 7cecd43..cc310a1 100644
--- a/test/tests/SingletonsTest.php
+++ b/test/tests/SingletonsTest.php
@@ -11,6 +11,7 @@
 use MLocati\ComuniItaliani\TerritoryWithChildren;
 use MLocati\ComuniItaliani\Region;
 use MLocati\ComuniItaliani\Province;
+use MLocati\ComuniItaliani\GeographicalSubdivision;
 
 class SingletonsTest extends TerritoryTestCase
 {
@@ -32,4 +33,52 @@ public function testSingletons(bool $unminified): void
         $geographicalSubdivisions = $factory->getGeographicalSubdivisions();
         $this->assertSame($geographicalSubdivisions, $factory->getGeographicalSubdivisions());
     }
+
+    /**
+     * @dataProvider provideGeographicalSubdivisions
+     */
+    public function testGeographicalSubdivision(GeographicalSubdivision $geographicalSubdivision): void
+    {
+        $this->assertNull($geographicalSubdivision->getParent());
+        $children = $geographicalSubdivision->getChildren();
+        $this->assertSame($children, $geographicalSubdivision->getRegions());
+        foreach ($children as $child) {
+            $this->assertSame($geographicalSubdivision, $child->getParent());
+        }
+    }
+
+    /**
+     * @dataProvider provideRegions
+     */
+    public function testRegion(Region $region): void
+    {
+        $this->assertInstanceOf(GeographicalSubdivision::class, $region->getParent());
+        $children = $region->getChildren();
+        $this->assertSame($children, $region->getProvinces());
+        foreach ($children as $child) {
+            $this->assertSame($region, $child->getParent());
+        }
+    }
+
+    /**
+     * @dataProvider provideProvinces
+     */
+    public function testProvince(Province $province): void
+    {
+        $this->assertInstanceOf(Region::class, $province->getParent());
+        $children = $province->getChildren();
+        $this->assertSame($children, $province->getMunicipalities());
+        foreach ($children as $child) {
+            $this->assertSame($province, $child->getParent());
+        }
+    }
+
+    /**
+     * @dataProvider provideMunicipalities
+     */
+    public function testMunicipality(Municipality $municipality): void
+    {
+        $this->assertInstanceOf(Province::class, $municipality->getParent());
+        $this->assertNotInstanceOf(TerritoryWithChildren::class, $municipality);
+    }
 }