Moving data from MS Access to MS SQL Server

I'm trying to migrate data into SQL Server from an existing legacy set of MS Access databases. Currently everything is working except a few core tables that are wide and contain a number of smalldatetime columns. The problem is an error that SQL Server reports when trying to move these tables:

Msg 8114, Level 16, State 8, Line 1 Error converting data type DBTYPE_DBTIMESTAMP to datetime.

I have a stored procedure which generates dynamic sql to assist with moving each table. I converted all the datetime tables in SQL Server to datetime2 columns, and I added a workaround, or so i thought, to deal with this datetime issue, but it doesn't seem to be working. the stored procedures are kind of bulky so the following is the relevant excerpt:

IF (@Conversion = 1 AND (@ColType LIKE '%date%' OR @ColType LIKE '%time%'))
    BEGIN
        SET @ColumnList = @ColumnList + ', IIF(ISDATE([' + @ColName + ']) = 1, [' + @ColName + '], CONVERT(datetime2, ''12/31/1899'')) AS ['+ @ColName + ']'
    END
ELSE
    BEGIN
        SET @ColumnList = @ColumnList + ', [' + @ColName + ']'
    END

Currently, the actual data that is "bad" (can't be converted) is not really important, so I'm trying to substitute it for an obviously invalid date just to get the process to complete without errors. The MS Access frontend has no code to prevent users from entering invalid dates, so for example if someone makes a typo and creates an order with the ship date of "june 2nd, 202", instead of "2015", MS Access will happily store this and then SQL Server fails to convert that value to a datetime.

Can anyone tell me what I'm doing wrong? I would appreciate it greatly.