Schema compare - 'Unable to cast object of type 'System.DBNull' to type 'System.String' (4268314)

User has a schema with many nicknames and stored procedures, as well as tables. (150 tables, 150 procedures, 150 nicknames and more than 200 indexes.
When doing a schema compare on one data warehouse to another, he encountered the error below:
'Unable to cast object of type 'System.DBNull' to type 'System.String'.

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Level 1 inner exception (Quest.Compare.Schema.Common.Pl.CNSLoader+ModelThreadException):
Unable to cast object of type 'System.DBNull' to type 'System.String'.
Call Stack:
at Quest.Compare.Schema.DB2.DBModel.CNS2RootManager.LoadModelForSchemaCompare(ICommandProgress progress, Boolean needReferences)
at Quest.Compare.Schema.Common.Pl.CNSCmdFillPairTree.CommandExecution(ICommandQueue icmdqueue)
at Quest.Compare.General.Sys.ProgressiveCommand.ExecuteCommand(ICommandQueue icmdqueue)


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Level 2 inner exception (System.InvalidCastException):
Unable to cast object of type 'System.DBNull' to type 'System.String'.
Call Stack:
at Quest.Compare.Schema.DB2.DBModel.CNS2Loader.DirectExtractReferencesIntoGraph(ICNSConnection iConnection, CNSPLProgress plProgress, LimitationsPack packLimitations, Boolean bIncludeSystemObjects, IReferenceGraph iGraph, Boolean useMultiThread)
at Quest.Compare.Schema.DB2.DBModel.CNS2Loader.CreateGraph(ICNSConnection iConnection, ICollection`1 arObjs, Boolean bDependent, Boolean bReferenced, Boolean bFirstLevelOnly, Boolean bothDepSublevels, ICNSRootManager iManager, IList`1 types, Boolean multiple)
at Quest.Compare.Schema.DB2.DBModel.CNS2Loader.CreateGraph(ICNSConnection iConnection, ICollection`1 arObjs, Boolean bDependent, Boolean bReferenced, Boolean bFirstLevelOnly, Boolean bothDepSublevels, ICNSRootManager iManager)
at Quest.Compare.Schema.DB2.DBModel.CNS2RootManager.LoadReferences(Object oFillingThreadParms)