maptostring returns null for very large long varbinary

I have a flex table with many different sizeslong varbinary items inserted ( 15 - 100,000 bytes). My problem is that I cannot apply maptostring on the very long ones - it is coming up null for the larger items in my table. I think the problem may be that they are longer than the max sized varchar. I have checked the actual text contained in these rows and they are showing 37000 characters of json strings as text, which would be greater than the maximum varchar. Any help would be appreciated.