diff --git a/frontend/modules/catalog/composables/__tests__/useProductForm.spec.ts b/frontend/modules/catalog/composables/__tests__/useProductForm.spec.ts
index 2c0b24a..9294f47 100644
--- a/frontend/modules/catalog/composables/__tests__/useProductForm.spec.ts
+++ b/frontend/modules/catalog/composables/__tests__/useProductForm.spec.ts
@@ -171,6 +171,20 @@ describe('useProductForm', () => {
             expect(payload.containsMolasses).toBe(false)
         })
 
+        it('omet `category` du payload quand aucune categorie n\'est choisie', async () => {
+            // Envoyer category:null casserait la denormalisation back (type IRI
+            // attendu) et court-circuiterait les autres violations -> on l'omet.
+            mockPost.mockResolvedValueOnce({ id: 40 })
+            const { form, submit } = useProductForm()
+            fillValidForm(form)
+            form.categoryIri = null
+
+            await submit()
+
+            const payload = mockPost.mock.calls[0][1]
+            expect(payload).not.toHaveProperty('category')
+        })
+
         it('mappe un 409 doublon de code sur errors.code + toast explicite', async () => {
             mockPost.mockRejectedValueOnce({ response: { status: 409, _data: {} } })
             const { form, errors, submit } = useProductForm()
diff --git a/frontend/modules/catalog/composables/useProductForm.ts b/frontend/modules/catalog/composables/useProductForm.ts
index 094fea5..0d6237b 100644
--- a/frontend/modules/catalog/composables/useProductForm.ts
+++ b/frontend/modules/catalog/composables/useProductForm.ts
@@ -126,7 +126,7 @@ export function useProductForm() {
         formErrors.clearErrors()
         const editing = productId.value !== null
         try {
-            const payload = {
+            const payload: Record<string, unknown> = {
                 code: form.code || null,
                 name: form.name || null,
                 states: form.states,
@@ -134,10 +134,17 @@ export function useProductForm() {
                 // re-force, on garde le payload coherent).
                 manufactured: isSale.value ? form.manufactured : false,
                 containsMolasses: isSale.value ? form.containsMolasses : false,
-                category: form.categoryIri,
                 sites: form.siteIris,
                 storageTypes: form.storageTypeIris,
             }
+            // `category` attend un IRI (string) : envoyer null declencherait une
+            // erreur de denormalisation API Platform qui court-circuiterait TOUTES
+            // les autres violations. On omet la cle quand aucune categorie n'est
+            // choisie -> la contrainte NotNull renvoie un message propre, et les
+            // autres champs sont valides dans la meme 422 (mapping inline ERP-101).
+            if (form.categoryIri) {
+                payload.category = form.categoryIri
+            }
             const options = { headers: { Accept: 'application/ld+json' }, toast: false }
             if (editing) {
                 await api.patch(`/products/${productId.value}`, payload, options)
diff --git a/src/Module/Catalog/Domain/Entity/Product.php b/src/Module/Catalog/Domain/Entity/Product.php
index 75605e7..7cc774b 100644
--- a/src/Module/Catalog/Domain/Entity/Product.php
+++ b/src/Module/Catalog/Domain/Entity/Product.php
@@ -81,12 +81,18 @@ use function in_array;
             security: "is_granted('catalog.products.manage')",
             normalizationContext: ['groups' => ['product:read', 'product:item:read', 'category:read', 'site:read', 'storage_type:read', 'default:read']],
             denormalizationContext: ['groups' => ['product:write']],
+            // Convertit les erreurs de denormalisation (type invalide / null sur une
+            // relation : category, sites, storageTypes) en violations 422 portant un
+            // propertyPath, au lieu d'un 400 qui court-circuite toute la validation
+            // (cf. Client/Supplier/WeighingTicket — mapping inline useFormErrors).
+            collectDenormalizationErrors: true,
             processor: ProductProcessor::class,
         ),
         new Patch(
             security: "is_granted('catalog.products.manage')",
             normalizationContext: ['groups' => ['product:read', 'product:item:read', 'category:read', 'site:read', 'storage_type:read', 'default:read']],
             denormalizationContext: ['groups' => ['product:write']],
+            collectDenormalizationErrors: true,
             provider: ProductProvider::class,
             processor: ProductProcessor::class,
         ),