Why does this always fail?

Failing Spec

using System;
using SolidWorks.Interop.sldworks;
using Xunit;

namespace SolidTp.SolidWorks.Domain.Tests {
	[Concern(typeof (IEquationManagerWriter))]
	public class Wenn_eine_variablendefinition_im_modell_aktualisiert_werden_soll_ : InstanceContextSpecification<IEquationManagerWriter> {
		private string _variableDefinitionAsEquation;

		protected override void EstablishContext() {
			The<IEquationMgr>().Equation[0] = "\"A11\"=10				' Schneidendurchmesser";
			The<IEquationMgr>().WhenToldTo(eqm => eqm.GetCount()).Return(1);
			_variableDefinitionAsEquation = "\"A11\"=12 ' Schneidendurchmesser";
			The<IVariableDefinition>().WhenToldTo(vd=>vd.AsEquation()).Return(_variableDefinitionAsEquation);
		}

		protected override IEquationManagerWriter CreateSut() {
			return new EquationManagerWriter(The<IEquationMgr>());
		}

		protected override void Because() {
			Sut.InsertOrUpdate(The<IVariableDefinition>());
		}

		[Observation]
		public void Sollte_die_gleichung_im_modell_die_neuen_informationen_enthalten() {
			The<IEquationMgr>().WasToldTo(mgr=>mgr.Equation[0] = _variableDefinitionAsEquation);
		}
	}

	public class EquationManagerWriter : IEquationManagerWriter {
		private readonly IEquationMgr _equationManager;

		public EquationManagerWriter(IEquationMgr equationManager) {
			_equationManager = equationManager;
		}

		public void InsertOrUpdate(IVariableDefinition variableDefinition) {
			int equationCount = _equationManager.GetCount();
			IEquationParser equationParser = new EquationParser();
			for (int i = 0; i < equationCount; i++) {
				if(null != equationParser.ParseVariableDefinition(_equationManager.Equation[i])) {
					_equationManager.Equation[i] = variableDefinition.AsEquation();
					break;
				}
			}
		}
	}

	public interface IEquationManagerWriter {
		void InsertOrUpdate(IVariableDefinition variableDefinition);
	}
}