diff --git a/src/backup/fixed_index.rs b/src/backup/fixed_index.rs
index bb3197ba..6dac8092 100644
--- a/src/backup/fixed_index.rs
+++ b/src/backup/fixed_index.rs
@@ -187,6 +187,9 @@ pub struct FixedIndexWriter {
     pub ctime: u64,
 }
 
+// `index` is mmap()ed which cannot be thread-local so should be sendable
+unsafe impl Send for FixedIndexWriter {}
+
 impl Drop for FixedIndexWriter {
 
     fn drop(&mut self) {