Cannot handle `dict.at(variable) = value` expression

[ntjess]

#let x = (a: 5)
#let key = "a"
#{
  x.at(key) = 6
}
#x

This is a valid typst document but pandoc produces the following error:

Cannot update expression FuncCall (FieldAccess (Ident (Identifier "at")) (Ident (Identifier "x"))) [NormalArg (Ident (Identifier "key"))]

Note that pandoc succeeds when directly indexing x with a string, i.e., x.at("a") = 6.