diff --git a/src/db.zig b/src/db.zig
index d197603..0f80a43 100644
--- a/src/db.zig
+++ b/src/db.zig
@@ -1670,6 +1670,14 @@ pub const Engine = struct {
             var coll_it = colls.iterator();
             while (coll_it.next()) |ce| {
                 const coll = ce.value_ptr.*;
+                // Everything below this line is written by a collection's own
+                // writer under its own lock, and `slab_extents` is an ArrayList
+                // that `slab_reserve` appends to -- so reading it under only the
+                // shared catalog lock could walk a slice a concurrent append had
+                // already reallocated. Lock order is catalog then collection,
+                // the same order `compact` uses.
+                try coll.lock.lockShared(self.io);
+                defer coll.lock.unlockShared(self.io);
                 try put_bytes(gpa, out, ce.key_ptr.*);
                 try put_u64(gpa, out, coll.slab_tail);
                 try put_u64(gpa, out, coll.slab_end);