Nullable range + array error

object.Exception@/usr/include/dmd/phobos/std/typecons.d(1181): Enforcement failed
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./test_nullable(pure @safe bool std.exception.enforce!(bool).enforce(bool, lazy const(char)[], immutable(char)[], ulong)+0x65) [0x41882d]
./test_nullable(std.typecons.Nullable!(int).Nullable.getinout(pure ref @property @safe inout(int) function())+0x89) [0x418759]
./test_nullable(std.typecons.Nullable!(int).Nullable* std.conv.emplace!(std.typecons.Nullable!(int).Nullable, std.typecons.Nullable!(int).Nullable).emplace(std.typecons.Nullable!(int).Nullable*, ref std.typecons.Nullable!(int).Nullable)+0x21) [0x419039]
./test_nullable(_D3std5array62__T5arrayTSmain34__T9MapResultS14main9__lambda2TAiZ9MapResultZ5arrayFSmain34__T9MapResultS14main9__lambda2TAiZ9MapResultZAS3std8typecons15__T8NullableTiZ8Nullable+0x7a) [0x418fbe]
./test_nullable(_Dmain+0x61) [0x418671]
./test_nullable(extern (C) int rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).void runMain()+0x18) [0x41a544]
./test_nullable(extern (C) int rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).void tryExec(scope void delegate())+0x2a) [0x41a082]
./test_nullable(extern (C) int rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).void runAll()+0x3b) [0x41a58b]
./test_nullable(extern (C) int rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).void tryExec(scope void delegate())+0x2a) [0x41a082]
./test_nullable(_d_run_main+0x1a8) [0x41a03c]
./test_nullable(main+0x17) [0x419e8f]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xed) [0x7f3c997be76d]
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