Thank you for your post.
This is usually an issue with the value it's returning. Could you send the calculation you have on the variable and any details needed to reproduce?
It was a simple substitute calculation
$Var2 = Substitute ( $Var1 ; "x" ; "y" )
But when I changed the value of "x" (I removed a "," from the expression), all went good.
But it is the first time I get this message, and even the Filemaker couldn't perform any script after this!