diff --git a/src/backtrace/miri.rs b/src/backtrace/miri.rs
index 036d39b4..6cfef10f 100644
--- a/src/backtrace/miri.rs
+++ b/src/backtrace/miri.rs
@@ -4,8 +4,8 @@ use std::vec::Vec;
 use std::boxed::Box;
 
 extern "Rust" {
-    fn miri_get_backtrace() -> Box<[*mut ()]>;
-    fn miri_resolve_frame(version: u8, ptr: *mut ()) -> MiriFrame;
+    fn miri_get_backtrace(flags: u64) -> Box<[*mut ()]>;
+    fn miri_resolve_frame(ptr: *mut (), flags: u64) -> MiriFrame;
 }
 
 #[derive(Debug)]
@@ -27,9 +27,8 @@ pub struct Frame {
     pub inner: MiriFrame
 }
 
-// Miri is single threaded, and the raw
-// pointer isn't really a pointer (it's a magic value
-// returned by Miri)
+// SAFETY: Miri guarantees that the returned pointer
+// can be used from any thread.
 unsafe impl Send for Frame {}
 unsafe impl Sync for Frame {}
 
@@ -52,13 +51,14 @@ impl Frame {
 }
 
 pub fn trace<F: FnMut(&super::Frame) -> bool>(cb: F) {
-    // Miri is single threaded
+    // SAFETY: Miri guarnatees that the backtrace API functions
+    // can be called from any thread.
     unsafe { trace_unsynchronized(cb) };
 }
 
 
 pub fn resolve_addr(ptr: *mut c_void) -> Frame {
-    let frame: MiriFrame = unsafe { miri_resolve_frame(0, ptr as *mut ()) };
+    let frame: MiriFrame = unsafe { miri_resolve_frame(ptr as *mut (), 0) };
     Frame {
         addr: ptr,
         inner: frame
@@ -66,7 +66,7 @@ pub fn resolve_addr(ptr: *mut c_void) -> Frame {
 }
 
 pub unsafe fn trace_unsynchronized<F: FnMut(&super::Frame) -> bool>(mut cb: F) {
-    let frames = miri_get_backtrace();
+    let frames = miri_get_backtrace(0);
     for ptr in frames.iter() {
         let frame = resolve_addr(*ptr as *mut c_void);
         cb(&super::Frame {