diff --git a/server/src/Casts/Polygon.php b/server/src/Casts/Polygon.php
index 181fd6107..7c1cb482f 100644
--- a/server/src/Casts/Polygon.php
+++ b/server/src/Casts/Polygon.php
@@ -32,18 +32,20 @@ public function get($model, $key, $value, $attributes)
     public function set($model, $key, $value, $attributes)
     {
         // Checked before the broader GeometryInterface guard below, which a
-        // SpatialPolygon also satisfies — otherwise this arm never fires. Both
-        // arms behave identically, so ordering does not change what is stored.
+        // SpatialPolygon also satisfies — otherwise this arm never fires.
+        // It must wrap in a SpatialExpression exactly as that guard does:
+        // returning the bare geometry here would change what is bound on writes
+        // that skip SpatialTrait::performInsert().
         if ($value instanceof SpatialPolygon) {
             $model->geometries[$key] = $value;
 
-            return $value;
+            return new SpatialExpression($value);
         }
 
         if ($value instanceof GeometryInterface) {
             $model->geometries[$key] = $value;
 
-            return $value;
+            return new SpatialExpression($value);
         }
 
         if (Utils::isGeoJson($value)) {
diff --git a/server/tests/Unit/Casts/SpatialCastBranchesTest.php b/server/tests/Unit/Casts/SpatialCastBranchesTest.php
index 4fb21d19e..7671a8f2e 100644
--- a/server/tests/Unit/Casts/SpatialCastBranchesTest.php
+++ b/server/tests/Unit/Casts/SpatialCastBranchesTest.php
@@ -38,12 +38,15 @@ function fleetopsSpatialCastSquare(): SpatialPolygon
     expect($returned)->toBeInstanceOf(SpatialExpression::class)
         ->and($model->geometries['border'])->toBe($multi);
 
-    // The polygon cast stashes the geometry but hands the value back directly
+    // The polygon cast wraps the geometry like every other spatial cast —
+    // returning the bare object lets class-cast re-serialisation overwrite the
+    // SpatialExpression that SpatialTrait::performInsert() prepared, so the
+    // raw geometry gets bound and MySQL rejects it with error 1416
     $polygonModel = new FleetOpsSpatialCastModel();
     $polygon      = fleetopsSpatialCastSquare();
     $polygonCast  = (new Fleetbase\FleetOps\Casts\Polygon())->set($polygonModel, 'border', $polygon, []);
 
-    expect($polygonCast)->toBe($polygon)
+    expect($polygonCast)->toBeInstanceOf(SpatialExpression::class)
         ->and($polygonModel->geometries['border'])->toBe($polygon);
 
     // Expressions are already bind-ready, so the point cast returns them