diff --git a/Modelica/Fluid/Vessels.mo b/Modelica/Fluid/Vessels.mo
index b3b746c4f4..a1b4ff9ea0 100644
--- a/Modelica/Fluid/Vessels.mo
+++ b/Modelica/Fluid/Vessels.mo
@@ -307,7 +307,7 @@ of the modeller. Increase nPorts to add an additional port.
         end for;
         // Check for correct solution
         assert(fluidLevel <= fluidLevel_max, "Vessel is overflowing (fluidLevel > fluidLevel_max = " + String(fluidLevel) + ")");
-        assert(fluidLevel > -1e-6*fluidLevel_max, "Fluid level (= " + String(fluidLevel) + ") is below zero meaning that the solution failed.");
+        assert(fluidLevel > 0, "Fluid level is too small, the tank cannot be completely empty otherwise the energy balance equation becomes singular.");
 
         // Boundary conditions
 
diff --git a/ModelicaTest/Fluid/TestComponents/Vessels/TestSimpleTank.mo b/ModelicaTest/Fluid/TestComponents/Vessels/TestSimpleTank.mo
index 5d93b1af49..449a82ff08 100644
--- a/ModelicaTest/Fluid/TestComponents/Vessels/TestSimpleTank.mo
+++ b/ModelicaTest/Fluid/TestComponents/Vessels/TestSimpleTank.mo
@@ -10,10 +10,10 @@ model TestSimpleTank
     height=1,
     nPorts=2,
     portsData={Modelica.Fluid.Vessels.BaseClasses.VesselPortsData(diameter=0.1,
-        height=0),Modelica.Fluid.Vessels.BaseClasses.VesselPortsData(diameter=
+        height=0.001),Modelica.Fluid.Vessels.BaseClasses.VesselPortsData(diameter=
         0.1, height=1)},
     crossArea=1,
-    level_start=0)
+    level_start=0.001)
     annotation (Placement(transformation(extent={{0,0},{40,40}})));
 
   inner Modelica.Fluid.System system(energyDynamics=Modelica.Fluid.Types.Dynamics.FixedInitial)