Not allowing zero sell amount orders

src/CoWSwapEthFlow.sol

@@ -86,6 +86,10 @@ contract CoWSwapEthFlow is
             revert IncorrectEthAmount();
         }
 
+        if (0 == order.sellAmount) {
+            revert NotAllowedZeroSellAmount();
+        }
+
         EthFlowOrder.OnchainData memory onchainData = EthFlowOrder.OnchainData(
             msg.sender,
             order.validTo

src/interfaces/ICoWSwapEthFlow.sol

@@ -13,6 +13,9 @@ interface ICoWSwapEthFlow {
     /// @dev Error thrown when trying to create an order without sending the expected amount of ETH to this contract.
     error IncorrectEthAmount();
 
+    /// @dev Error thrown when trying to create an order with a sell amount == 0
+    error NotAllowedZeroSellAmount();
+
     /// @dev Error thrown if trying to delete an order while not allowed.
     error NotAllowedToDeleteOrder(bytes32 orderHash);
 

test/CoWSwapEthFlow.t.sol

@@ -175,6 +175,26 @@ contract TestOrderCreation is EthFlowTestSetup, ICoWSwapOnchainOrders {
         ethFlow.createOrder{value: sellAmount + feeAmount - 1}(order);
     }
 
+    function testRevertOrderCreationIfSellAmountIsZero() public {
+        uint256 sellAmount = 0 ether;
+        uint256 feeAmount = 1 ether;
+        EthFlowOrder.Data memory order = EthFlowOrder.Data(
+            IERC20(address(0)),
+            address(0),
+            sellAmount,
+            0,
+            bytes32(0),
+            feeAmount,
+            0,
+            false,
+            0
+        );
+        assertEq(order.sellAmount, sellAmount);
+
+        vm.expectRevert(ICoWSwapEthFlow.NotAllowedZeroSellAmount.selector);
+        ethFlow.createOrder{value: sellAmount + feeAmount}(order);
+    }
+
     function testRevertIfCreatingAnOrderWithTheSameHashTwice() public {
         uint256 sellAmount = 41 ether;
         uint256 feeAmount = 1 ether;