diff --git a/src/domain/work-orders/mappers/work-order-board-patch-mapper.ts b/src/domain/work-orders/mappers/work-order-board-patch-mapper.ts
index 87d68f25..c5714dc6 100644
--- a/src/domain/work-orders/mappers/work-order-board-patch-mapper.ts
+++ b/src/domain/work-orders/mappers/work-order-board-patch-mapper.ts
@@ -84,7 +84,7 @@ export function buildBoardPatchPayload(
     if (row.dispatchRowVersion) {
       payload.dispatchVersion = row.dispatchRowVersion;
     }
-    if (row.primaryDispatchId != null) {
+    if (row.primaryDispatchId != null && row.primaryDispatchId > 0) {
       payload.primaryDispatchId = row.primaryDispatchId;
     }
   }
diff --git a/src/test/domain/work-orders/mappers/work-order-board-patch-mapper.test.ts b/src/test/domain/work-orders/mappers/work-order-board-patch-mapper.test.ts
index 3aae3dc6..9975fc43 100644
--- a/src/test/domain/work-orders/mappers/work-order-board-patch-mapper.test.ts
+++ b/src/test/domain/work-orders/mappers/work-order-board-patch-mapper.test.ts
@@ -205,6 +205,25 @@ describe("buildBoardPatchPayload", () => {
     });
   });
 
+  it("omits non-positive primaryDispatchId on dispatch fields", () => {
+    const payload = buildBoardPatchPayload(
+      { field: "vendorId", value: "7" },
+      {
+        rowVersion: "wo-v1",
+        dispatchRowVersion: "disp-v1",
+        primaryDispatchId: 0,
+      },
+    );
+
+    expect(payload).toEqual({
+      field: "vendorId",
+      value: "7",
+      workOrderVersion: "wo-v1",
+      dispatchVersion: "disp-v1",
+    });
+    expect(payload).not.toHaveProperty("primaryDispatchId");
+  });
+
   it("omits dispatch version for WO-only fields", () => {
     const payload = buildBoardPatchPayload(
       { field: "siteCode", value: "BK5" },
diff --git a/src/test/domain/work-orders/use-cases/use-work-order-table-mutations.test.tsx b/src/test/domain/work-orders/use-cases/use-work-order-table-mutations.test.tsx
index 68b694fe..e34a7cfd 100644
--- a/src/test/domain/work-orders/use-cases/use-work-order-table-mutations.test.tsx
+++ b/src/test/domain/work-orders/use-cases/use-work-order-table-mutations.test.tsx
@@ -439,6 +439,7 @@ describe("useWorkOrderTableMutations vendor assignment round-trip", () => {
     );
     const payload = patchBoardField.mock.calls[0]?.[1] as Record<string, unknown>;
     expect(payload).not.toHaveProperty("primaryDispatchStatus");
+    expect(payload.primaryDispatchId).toBe(10);
     expect(returnedRow.primaryDispatchId).not.toBe(10);
     expect(getClosabilityGaps(tableRowToClosabilityInput(returnedRow))).not.toContain("Company");
   });
@@ -484,6 +485,50 @@ describe("useWorkOrderTableMutations vendor assignment round-trip", () => {
     );
     expect(getClosabilityGaps(tableRowToClosabilityInput(returnedRow))).toContain("Company");
   });
+
+  it("omits non-positive primaryDispatchId from the vendor PATCH payload", async () => {
+    rowState = { ...rowState, primaryDispatchId: 0 };
+    const returnedRow = mapWorkOrderTableRow({
+      id: 42,
+      VendorId: 9,
+      VendorName: "New Co",
+      PrimaryDispatchStatus: "Pending",
+      PrimaryDispatchId: 11,
+    });
+    patchBoardField.mockResolvedValue(returnedRow);
+
+    const onPatch = vi.fn();
+    const { result } = renderHook(
+      () =>
+        useWorkOrderTableMutations({
+          onPatch,
+          clearPatch: () => undefined,
+          getRow: () => rowState,
+        }),
+      { wrapper: makeWrapper() },
+    );
+
+    act(() => {
+      result.current.patchField(
+        42,
+        toVendorTablePatch({
+          vendorId: "9",
+          company: "New Co",
+          tech: "",
+          techPhone: "",
+        }),
+      );
+    });
+
+    await waitFor(() => expect(patchBoardField).toHaveBeenCalled());
+    const payload = patchBoardField.mock.calls[0]?.[1] as Record<string, unknown>;
+    expect(payload).toMatchObject({
+      field: "vendorId",
+      value: "9",
+      dispatchVersion: "disp-v1",
+    });
+    expect(payload).not.toHaveProperty("primaryDispatchId");
+  });
 });
 
 describe("useWorkOrderTableMutations patchField success callbacks", () => {