diff --git a/API/Services/Extensions.cs b/API/Services/Extensions.cs
index 32e704f..34b5022 100644
--- a/API/Services/Extensions.cs
+++ b/API/Services/Extensions.cs
@@ -214,7 +214,7 @@ public static Graph ToGraph(this Models.Soil soil, double[] thickness = null,
                                    .Multiply(soil.Water.Thickness).Sum();
 
         // To stop the sw line crossing over ll, constrain it to ll.
-        var swConstainedToLL = sw.LowerConstraint(ll);
+        var swConstainedToLL = sw?.LowerConstraint(ll);
         return SoilGraph.Create(soil.Name, soil.Water.Thickness.ToMidPoints(), soil.Water.AirDry, soil.Water.LL15,
                                     soil.Water.DUL, soil.Water.SAT, ll, cropName, pawc, paw, thickness?.ToMidPoints(), swConstainedToLL);
     }
diff --git a/Tests/UnitTest.cs b/Tests/UnitTest.cs
index ed268d7..ab84bdc 100644
--- a/Tests/UnitTest.cs
+++ b/Tests/UnitTest.cs
@@ -286,4 +286,17 @@ public void ToAPSIMSoil_ShouldPopulateAllMembers()
         var apsimSoilXml = soil.ToAPSIMSoil().ToXML();
         Assert.That(apsimSoilXml, Is.EqualTo(ResourceFile.Get("Tests.testsoil1.apsim.xml")));
     }
+
+    [Test]
+    public void ToGraph_ShouldWorkWithoutSWSpecified()
+    {
+        var soil = ResourceFile.FromResourceXML<API.Models.Soil>("Tests.testsoil1.xml");
+
+        var graph = soil.ToGraph();
+        Assert.That(graph, Is.Not.Null);
+
+        // should be 5 graph series:
+        //  shaded bucket, ll, airdry, dul, sat
+        Assert.That(graph.Series.Count, Is.EqualTo(5));
+    }
 }
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