Enterprise library Issue with null exception.

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database

Microsoft

data

I am getting this error frequently when no of users are more in production server.
The issue is resolved after getting IIS Restart.Using enterprise library 4.0.can someone help me on this issue.I am not getting the error in local machine.If you restart the IIS than everything will work fine.Now every two hour I am restarting the IIS of production server.

error massage

Microsoft.Practices.EnterpriseLibrary.Data.Database.PrepareCommand(DbCommand command, DbConnection connection)
   at Microsoft.Practices.EnterpriseLibrary.Data.Database.PrepareCommand(DbCommand command, DbTransaction transaction)
   at Microsoft.Practices.EnterpriseLibrary.Data.Database.ExecuteNonQuery(DbCommand command, DbTransaction transaction)
   at Lera.Dal.DataService.ExecuteNonQueryByStoredProc(String storedProcedureName, Object[] parameterValues) in C:\Projects\ISCSA\PROD\ISCSA_Source Code_B\Shared\Lera.Dal\DataService.cs:line 128
   at Lera.IPCSA.DAO.DaoService`1.Save(T entity, String query) in C:\Projects\ISCSA\PROD\ISCSA_Source Code_B\IPCSA\Lera.IPCSA.DAO\DaoService.cs:line 58
   at Lera.IPCSA.DAO.AssessmentStatusTrackerDao.SaveAssessmentStatusTracker(AssessmentStatusTrackerInfo assessmentStatusTracker) in C:\Projects\ISCSA\PROD\ISCSA_Source Code_B\IPCSA\Lera.IPCSA.DAO\AssessmentStatusTrackerDao.cs:line 32
   at Lera.IPCSA.Business.AssessmentStatusTrackerBL.SaveAssessmentStatusTracker(AssessmentStatusTrackerInfo assessmentStatusTracker) in C:\Projects\ISCSA\PROD\ISCSA_Source Code_B\IPCSA\Lera.IPCSA.Business\AssessmentStatusTrackerBL.cs:line 21
   at Lera.IPCSA.UI.Web.Assessment.AssessmentLevelDetails.SaveAssessmentStatusTracker(String Status) in C:\Projects\ISCSA\PROD\ISCSA_Source Code_B\IPCSA\Lera.IPCSA.UI.Web\Assessment\AssessmentLevelDetails.aspx.cs:line 1228
   at Lera.IPCSA.UI.Web.Assessment.AssessmentLevelDetails.SaveAssessment(Boolean isConstructRequired, String Status) in C:\Projects\ISCSA\PROD\ISCSA_Source Code_B\IPCSA\Lera.IPCSA.UI.Web\Assessment\AssessmentLevelDetails.aspx.cs:line 1036
   at Lera.IPCSA.UI.Web.Assessment.AssessmentLevelDetails.NavigateSubDomain(Boolean navigateNextSubDomain, Boolean isFromButtons, Boolean isDomainChange) in C:\Projects\ISCSA\PROD\ISCSA_Source Code_B\IPCSA\Lera.IPCSA.UI.Web\Assessment\AssessmentLevelDetails.aspx.cs:line 813
   at Lera.IPCSA.UI.Web.Assessment.AssessmentLevelDetails.btnNext_Click(Object sender, EventArgs e) in C:\Projects\ISCSA\PROD\ISCSA_Source Code_B\IPCSA\Lera.IPCSA.UI.Web\Assessment\AssessmentLevelDetails.aspx.cs:line 786
   at System.Web.UI.WebControls.Button.OnClick(EventArgs e)
   at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

The code i was executing is

public DataSet GetDataSetByStoredProc(string storedProcedureName, params object[] parameterValues)
        {
            DbCommand command = Database.GetStoredProcCommand(storedProcedureName);
           
            foreach (object parameterValue in parameterValues)
            {
                command.Parameters.Add(parameterValue);
            }

            return Database.ExecuteDataSet(command);
        }