[1777042] Error walking route

OK, it all tests OK until I run the sync simulation. If an object has a NULL value in the ID column, an exception is thrown. Inside the target system I can see the problem objects with this query:

select * from employee_master
where ISNULL(country_id,0)=0  OR ISNULL(region_id,0)=0

In the sync, the error that is thrown looks like this:

[1777239] The mapping rule (Spare field no. 08 <-> vrtObj_CountryID_Code) was unable to execute synchronization between system objects (1750) and (1750) successfully!
    at System.Threading.Tasks.Task.<>c__DisplayClass11.<ExecuteSelfReplicating>b__10(Object param0)
    at System.Threading.Tasks.Task.InnerInvokeWithArg(Task childTask)
    at System.Threading.Tasks.Parallel.<>c__DisplayClassf`1.<ForWorker>b__c()
    at VI.Projector.Mapping.Extensions.<>c__DisplayClass4_0.<MapObjects>b__0(SystemObjectMatchingPair pair)
    at VI.Projector.Mapping.SystemMappingRule.ApplyMapping(ISystemObject leftObject, ISystemObject rightObject, SystemMappingOptions options)
[1777160] Property (vrtObj_CountryID_Code@dbo.employee_master) could not read the value from system object (1750)!
    at VI.Projector.Mapping.SystemMappingRule.ApplyMapping(ISystemObject leftObject, ISystemObject rightObject, SystemMappingOptions options)
    at VI.Projector.Mapping.Rules.SinglePropertyComparisonRule.OnApplyMapping(ISystemObject leftObject, ISystemObject rightObject, SystemMappingOptions options)
    at VI.Projector.Mapping.Rules.SinglePropertyComparisonRule._GetValue(ISystemObject systemObject, ISchemaProperty property, IProjectorProcessContext context, Boolean useOldValues, Boolean handleAsSingleValue)
    at VI.Projector.Mapping.Rules.SinglePropertyComparisonRule.<>c__DisplayClass14_0.<_GetValue>b__0()
    at VI.Projector.Schema.SchemaProperty.GetValue(ISystemObject systemObject, SchemaPropertyGetValueOption option)
[1777042] Error walking route (VRT_country_idAsString.code)!
    at VI.Projector.Schema.SchemaProperty.GetValue(ISystemObject systemObject, SchemaPropertyGetValueOption option)
    at VI.Projector.Schema.SchemaWalker.get_Item(String route, Object defaultValue)
[1777027] Error loading a system object using the key property "country_id@dbo.countries"!
    at VI.Projector.Schema.SchemaWalker.get_Item(String route, Object defaultValue)
    at VI.Projector.Schema.SchemaWalker.OnWalk(String[] route)
    at VI.Projector.Schema.SchemaWalker._QueryData(ISystemConnection connection, ISchemaProperty keyProperty, Object keyValue, ISchemaProperty valueProperty)
    at VI.Projector.Connection.SystemConnection.QueryObject(ISchemaProperty keyProperty, Object value, QueryByKeyOptions query)
Specified cast is not valid.
    at VI.Projector.Connection.SystemConnection.QueryObject(ISchemaProperty keyProperty, Object value, QueryByKeyOptions query)
    at VI.Projector.Connection.Connectors.DistributionConnector.QueryObject(QueryObjectByKeyRequest request)
    at VI.Projector.Connection.SystemConnector.QueryObject(QueryObjectByKeyRequest request)
    at VI.Projector.Database.Native.NativeDatabaseConnector.OnQueryObject(ISchemaProperty keyProperty, Object value, QueryObjectByKeyRequest request)
    at VI.Projector.Database.Native.Access.SqlBasedImplementedSqlCodeFormatterBase.OnFormatComparison(NativeDatabaseSchemaProperty property, Object value, FilterCompareOperator compareOperator)
    at VI.Projector.Database.Native.Access.SqlBasedImplementedSqlCodeFormatterBase.OnFormatValue(NativeDatabaseSchemaProperty property, Object value, FormatValueOption option)

Any advice on how to get the object reference to just skip if the value is NULL? I already have conditions in place so the mapping is only set if the ID has a numerical value.

OK, it all tests OK until I run the sync simulation. If an object has a NULL value in the ID column, an exception is thrown. Inside the target system I can see the problem objects with this query:

select * from employee_master
where ISNULL(country_id,0)=0  OR ISNULL(region_id,0)=0

In the sync, the error that is thrown looks like this:

[1777239] The mapping rule (Spare field no. 08 <-> vrtObj_CountryID_Code) was unable to execute synchronization between system objects (1750) and (1750) successfully!
    at System.Threading.Tasks.Task.<>c__DisplayClass11.<ExecuteSelfReplicating>b__10(Object param0)
    at System.Threading.Tasks.Task.InnerInvokeWithArg(Task childTask)
    at System.Threading.Tasks.Parallel.<>c__DisplayClassf`1.<ForWorker>b__c()
    at VI.Projector.Mapping.Extensions.<>c__DisplayClass4_0.<MapObjects>b__0(SystemObjectMatchingPair pair)
    at VI.Projector.Mapping.SystemMappingRule.ApplyMapping(ISystemObject leftObject, ISystemObject rightObject, SystemMappingOptions options)
[1777160] Property (vrtObj_CountryID_Code@dbo.employee_master) could not read the value from system object (1750)!
    at VI.Projector.Mapping.SystemMappingRule.ApplyMapping(ISystemObject leftObject, ISystemObject rightObject, SystemMappingOptions options)
    at VI.Projector.Mapping.Rules.SinglePropertyComparisonRule.OnApplyMapping(ISystemObject leftObject, ISystemObject rightObject, SystemMappingOptions options)
    at VI.Projector.Mapping.Rules.SinglePropertyComparisonRule._GetValue(ISystemObject systemObject, ISchemaProperty property, IProjectorProcessContext context, Boolean useOldValues, Boolean handleAsSingleValue)
    at VI.Projector.Mapping.Rules.SinglePropertyComparisonRule.<>c__DisplayClass14_0.<_GetValue>b__0()
    at VI.Projector.Schema.SchemaProperty.GetValue(ISystemObject systemObject, SchemaPropertyGetValueOption option)
[1777042] Error walking route (VRT_country_idAsString.code)!
    at VI.Projector.Schema.SchemaProperty.GetValue(ISystemObject systemObject, SchemaPropertyGetValueOption option)
    at VI.Projector.Schema.SchemaWalker.get_Item(String route, Object defaultValue)
[1777027] Error loading a system object using the key property "country_id@dbo.countries"!
    at VI.Projector.Schema.SchemaWalker.get_Item(String route, Object defaultValue)
    at VI.Projector.Schema.SchemaWalker.OnWalk(String[] route)
    at VI.Projector.Schema.SchemaWalker._QueryData(ISystemConnection connection, ISchemaProperty keyProperty, Object keyValue, ISchemaProperty valueProperty)
    at VI.Projector.Connection.SystemConnection.QueryObject(ISchemaProperty keyProperty, Object value, QueryByKeyOptions query)
Specified cast is not valid.
    at VI.Projector.Connection.SystemConnection.QueryObject(ISchemaProperty keyProperty, Object value, QueryByKeyOptions query)
    at VI.Projector.Connection.Connectors.DistributionConnector.QueryObject(QueryObjectByKeyRequest request)
    at VI.Projector.Connection.SystemConnector.QueryObject(QueryObjectByKeyRequest request)
    at VI.Projector.Database.Native.NativeDatabaseConnector.OnQueryObject(ISchemaProperty keyProperty, Object value, QueryObjectByKeyRequest request)
    at VI.Projector.Database.Native.Access.SqlBasedImplementedSqlCodeFormatterBase.OnFormatComparison(NativeDatabaseSchemaProperty property, Object value, FilterCompareOperator compareOperator)
    at VI.Projector.Database.Native.Access.SqlBasedImplementedSqlCodeFormatterBase.OnFormatValue(NativeDatabaseSchemaProperty property, Object value, FormatValueOption option)

Any advice on how to get the object reference to just skip if the value is NULL? I already have conditions in place so the mapping is only set if the ID has a numerical value.