diff --git a/src/Domain.LinnApps/PurchaseOrders/PurchaseOrderDetail.cs b/src/Domain.LinnApps/PurchaseOrders/PurchaseOrderDetail.cs
index 4f1eb2fc5..d53d519fc 100644
--- a/src/Domain.LinnApps/PurchaseOrders/PurchaseOrderDetail.cs
+++ b/src/Domain.LinnApps/PurchaseOrders/PurchaseOrderDetail.cs
@@ -130,7 +130,12 @@ public bool ValidPrices()
             {
                 return false;
             }
-            
+
+            if (this.BaseOrderUnitPrice < 0)
+            {
+                return false;
+            }
+
             return true;
         }
 
diff --git a/tests/Unit/Domain.LinnApps.Tests/PurchaseOrderServiceTests/WhenUpdatingAndNegativeBaseUnitPrice.cs b/tests/Unit/Domain.LinnApps.Tests/PurchaseOrderServiceTests/WhenUpdatingAndNegativeBaseUnitPrice.cs
new file mode 100644
index 000000000..a1da723e9
--- /dev/null
+++ b/tests/Unit/Domain.LinnApps.Tests/PurchaseOrderServiceTests/WhenUpdatingAndNegativeBaseUnitPrice.cs
@@ -0,0 +1,63 @@
+namespace Linn.Purchasing.Domain.LinnApps.Tests.PurchaseOrderServiceTests
+{
+    using System;
+    using System.Collections.Generic;
+
+    using FluentAssertions;
+
+    using Linn.Purchasing.Domain.LinnApps.Exceptions;
+    using Linn.Purchasing.Domain.LinnApps.PurchaseLedger;
+    using Linn.Purchasing.Domain.LinnApps.PurchaseOrders;
+
+    using NSubstitute;
+
+    using NUnit.Framework;
+
+    public class WhenUpdatingAndNegativeBaseUnitPrice : ContextBase
+    {
+        private readonly int orderNumber = 600179;
+
+        private PurchaseOrder updated;
+
+        private Action act;
+
+        [SetUp]
+        public void SetUp()
+        {
+            this.updated = new PurchaseOrder
+                               {
+                                   OrderNumber = this.orderNumber,
+                                   Details =
+                                       new List<PurchaseOrderDetail>
+                                           {
+                                               new PurchaseOrderDetail
+                                                   {
+                                                       Cancelled = "N",
+                                                       Line = 1,
+                                                       BaseNetTotal = 100m,
+                                                       NetTotalCurrency = 120m,
+                                                       BaseOrderUnitPrice = -100m,
+                                                       OrderNumber = this.orderNumber,
+                                                       OurQty = 99m,
+                                                       OrderQty = 12m,
+                                                       PartNumber = "P"
+                                                   }
+                                           }
+                               };
+
+            this.MockAuthService.HasPermissionFor(AuthorisedAction.PurchaseOrderUpdate, Arg.Any<IEnumerable<string>>())
+                .Returns(true);
+
+            this.PurchaseLedgerMaster.GetRecord().Returns(new PurchaseLedgerMaster { OkToRaiseOrder = "Y" });
+
+            this.act = () => this.Sut.UpdateOrder(new PurchaseOrder(), this.updated, new List<string>());
+        }
+
+
+        [Test]
+        public void ShouldThrow()
+        {
+            this.act.Should().Throw<PurchaseOrderException>("Prices must be positive numbers");
+        }
+    }
+}