Changing the transformation script to this works

return int64($1) / (1024 * 1024);

DCI in question is Filesystem.Free(/var)

Transformation script after trial and error

trace(0, $node->name . " " . $dci->name . " " . $1 . " " . typeof($1));
return uint64($1 / (1024 * 1024));

Relevant part from log when above DCI is polled

2017.12.12 03:57:13.034 *I* alina FileSystem.Free(/var) 58124193792 uint64
2017.12.12 03:57:13.034 *W* Failed to execute transformation script for object 865 "alina" DCI 813 "FileSystem.Free(/var)": Error 13 in line 2: Cannot do automatic type cast

Also tried without casting to uint64.

Changing the transformation script to return null (which from my understanding of the hint should return the raw value) results in free space being reported as 0. Actual value printed to log for reference.
Transformation script

trace(0, $node->name . " " . $dci->name . " " . $1 . " " . typeof($1));
return null;
return uint64($1 / (1024 * 1024));

Log

2017.12.12 04:07:10.221 *I* alina FileSystem.Free(/var) 58124161024 uint64

Transformation script before update was simply

$1 / (1024 * 1024)

Clearing out the transformation script returns the raw value.

These are Unsigned INT64

Also, I am finding (perhaps related) when I do an 'walk' on any SNMP object, I get: "Cannot do SNMP MIB tree walk: Access denied" and it worked before...