diff --git a/src/vmm_ring.ml b/src/vmm_ring.ml
index 90ca65f..26ec4df 100644
--- a/src/vmm_ring.ml
+++ b/src/vmm_ring.ml
@@ -19,30 +19,37 @@ let write t entry =
 
 let dec t n = (pred n + t.size) mod t.size
 
+let get_valid t idx =
+  let our = Array.get t.data idx in
+  if Ptime.equal (fst our) Ptime.min then
+    None
+  else
+    Some our
+
 let read_last t ?(tst = fun _ -> true) n =
   let rec one idx count acc =
-    let our = Array.get t.data idx in
-    if tst (snd our) then
-      if pred count = 0 then
-        our :: acc
-      else
-        one (dec t idx) (pred count) (our :: acc)
-    else
-      one (dec t idx) count acc
+    if count = 0 then
+      acc
+    else match get_valid t idx with
+      | None -> acc
+      | Some our ->
+        if tst (snd our) then
+          one (dec t idx) (pred count) (our :: acc)
+        else
+          one (dec t idx) count acc
   in
   one (dec t t.write) n []
 
 let read_history t ?(tst = fun _ -> true) since =
-  let rec go acc idx =
-    let entry = Array.get t.data idx in
-    if Ptime.equal (fst entry) Ptime.min then
-      acc
-    else if tst (snd entry) then
+  let rec go idx acc =
+    match get_valid t idx with
+    | None -> acc
+    | Some entry ->
       if Ptime.is_earlier (fst entry) ~than:since then
         acc
+      else if tst (snd entry) then
+        go (dec t idx) (entry :: acc)
       else
-        go (entry :: acc) (dec t idx)
-    else
-      go acc (dec t idx)
+        go (dec t idx) acc
   in
-  go [] (dec t t.write)
+  go (dec t t.write) []