Bug when using 'stable attribute

[Schottkyc137]

Sorry for opening up so many issues lately 😬
Related to #495 ?

Name       WORK.TB.ARRAY_SIG$stable
Kind       function
Context    WORK.TB
Blocks     1
Registers  3
Types      4
Variables  0
Result     0..1
Parameters 1
  r0    context                         // P<WORK.TB>
Begin
   0: r1 := var upref 1, ARRAY_SIG      // @<WORK.TB-ARCH.T_RECORD${}> => WORK.TB-ARCH.T_RECORD${}
      r2 := const 2                     // # => 2
      invalid := event r1 length r2	 <----

** Fatal: argument to event is not a signal
[0x5acbe2eae1b6] ../src/diag.c:1048 diag_femit
[0x5acbe2eae1b6] ../src/diag.c:1073 diag_emit
[0x5acbe2dce024] ../src/util.c:585 fatal_trace
[0x5acbe2ddcc1d] ../src/vcode.c:5068 emit_signal_flag.lto_priv.0.cold
[0x5acbe2e8e7e1] ../src/vcode.c:5076 lower_decls.lto_priv.0
[0x5acbe2e49b39] ../src/lower.c:13108 lower_instance
[0x5acbe2e49b39] ../src/lower.c:1415 elab_lower.lto_priv.0
[0x5acbe2e4bae1] ../src/elab.c:1535 elab_architecture.lto_priv.0
[0x5acbe2debe83] ../src/elab.c:2302 elab
[0x5acbe2debe83] ../src/elab.c:465 elaborate
[0x5acbe2de8ca9] ../src/nvc.c:2088 process_command
[0x5acbe2de8558] ../src/nvc.c:1772 cover_report_cmd
[0x5acbe2de8558] ../src/nvc.c:2118 process_command
[0x5acbe2de4c66] ../src/nvc.c:2230 main

nvc 1.14-devel (1.13.0.r100.ge821b31a) (Using LLVM 14.0.0) [x86_64-pc-linux-gnu]

Please report this bug at https://github.com/nickg/nvc/issues

VHDL input:

entity tb is
end entity tb;

architecture arch of tb is
    type T_RECORD is record
        EL : bit;
    end record T_RECORD;

    type T_ARRAY is array (1 downto 0) of T_RECORD;

    signal array_sig : T_ARRAY;

    signal stable_sig : boolean;
begin
    stable_sig <= array_sig'stable;
end architecture arch;

seems to only happen with the 'stable attribute. Using, for example, 'active works as intended.