While profiling an app with Studio collect using the debug build of b166, I got this assert:

# Internal Error (/scratch/HUDSON/workspace/9-2-build-linux-amd64-phase2/jdk9/6322/hotspot/src/cpu/x86/vm/frame_x86.cpp:699), pid=9520, tid=9695
# assert(_last_Java_pc == NULL) failed: already walkable

Stack: [0x00007f01a0de8000,0x00007f01a0ee9000], sp=0x00007f01a0ee03d0, free space=992k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x16d2c92] VMError::report_and_die(int, char const*, char const*, __va_list_tag*, Thread*, unsigned char*, void*, void*, char const*, int, unsigned long)+0x162
V [libjvm.so+0x16d3a1f] VMError::report_and_die(Thread*, char const*, int, char const*, char const*, __va_list_tag*)+0x2f
V [libjvm.so+0xaa382d] report_vm_error(char const*, int, char const*, char const*, ...)+0xdd
V [libjvm.so+0xbf0136] frame::sender_for_entry_frame(RegisterMap*) const+0x1b6
V [libjvm.so+0xbf1560] frame::sender(RegisterMap*) const+0x2f0
V [libjvm.so+0xe35166] java_lang_Throwable::fill_in_stack_trace(Handle, methodHandle const&, Thread*)+0x416
V [libjvm.so+0xe35c03] java_lang_Throwable::fill_in_stack_trace(Handle, methodHandle const&)+0x63
V [libjvm.so+0xf42991] JVM_FillInStackTrace+0x111
C [libjava.so+0x13ff1] Java_java_lang_Throwable_fillInStackTrace+0x11