Interface Definition: Expression evaluation error [evaluation ID = 2ce92:fe3b0] : An error occurred while executing a save: java.lang.IllegalArgumentException: Cannot derive list type from List of Text String in which to store data of type List of Variant

I am trying to save data in the submit button from CDT from interface into another array CDT where i use

a!save(ri!big_cdt.subset1, a!foreach( items:ri!samll_cdt, expression: {fv!item.subset1}))...

this way i am filling multiple values so that i can write array CDT into a datastore as multiple rows in an single attempt using write to data store in the process model.

but, i get this error in the interface "Interface Definition: Expression evaluation error [evaluation ID = 2ce92:fe3b0] : An error occurred while executing a save: java.lang.IllegalArgumentException: Cannot derive list type from List of Text String in which to store data of type List of Variant"

not sure what i am missing ?