the whole backtrace

$ perl6 -e 'sub foo(*@a) { @a[+0] }; foo()'
Cannot substr on a null string
current instr.: 'perl6;Any;substr' pc 392639 (src/gen/core.pir:39216)
called from Sub 'perl6;Signature;_block22064' pc 490745 (src/gen/core.pir:74262)
called from Sub 'perl6;Signature;_block22012' pc 489600 (src/gen/core.pir:73755)
called from Sub 'perl6;Signature;_block21990' pc 489362 (src/gen/core.pir:0)
called from Sub 'perl6;GatherIterator;' pc 512916 (src/gen/core.pir:82140)
called from Sub 'perl6;GatherIterator;get' pc 512896 (src/gen/core.pir:82135)
called from Sub 'perl6;Iterator;eager' pc 10834 (src/builtins/Role.pir:143)
called from Sub 'perl6;Seq;!STORE' pc 14241 (src/builtins/Routine.pir:76)
called from Sub 'perl6;Signature;perl' pc -1 ((unknown file):-1)
called from Sub '!postcircumfix:<[ ]>' pc 10733 (src/builtins/Role.pir:90)
... call repeated 1 times
called from Sub 'foo' pc 110 (EVAL_1:51)
called from Sub '_block14' pc 29 (EVAL_1:0)
called from Sub '!YOU_ARE_HERE' pc 359519 (src/gen/core.pir:25918)
called from Sub '!UNIT_START' pc 1299 (src/glue/run.pir:24)
called from Sub 'perl6;PCT;HLLCompiler;eval' pc -1 ((unknown file):-1)
called from Sub 'perl6;PCT;HLLCompiler;command_line' pc 1504 (compilers/pct/src/PCT/HLLCompiler.pir:801)
called from Sub 'perl6;Perl6;Compiler;main' pc -1 ((unknown file):-1)