diff --git a/lib/cachet.ml b/lib/cachet.ml
index 06cb03c..89a26aa 100644
--- a/lib/cachet.ml
+++ b/lib/cachet.ml
@@ -37,6 +37,8 @@ let memmove src ~src_off dst ~dst_off ~len =
   let dst = Bigarray.Array1.sub dst dst_off len in
   Bigarray.Array1.blit src dst
 
+let invalid_argf fmt = Format.kasprintf invalid_arg fmt
+
 module Bstr = struct
   type t = bigstring
 
@@ -396,7 +398,6 @@ external hash : (int32[@unboxed]) -> int -> (int32[@unboxed])
 [@@noalloc]
 
 let hash h d = Int32.to_int (hash h d)
-let failwithf fmt = Format.ksprintf (fun str -> failwith str) fmt
 
 type slice = { offset: int; length: int; payload: bigstring }
 
@@ -480,7 +481,8 @@ let load t ?(len = 1) logical_address =
   if len > 1 lsl t.pagesize then
     invalid_arg "Cachet.load: you can not load more than a page";
   if logical_address < 0 then
-    invalid_arg "Cachet.load: a logical address must be positive";
+    invalid_argf "Cachet.load: a logical address must be positive (%08x)"
+      logical_address;
   let page = logical_address lsr t.pagesize in
   let hash = hash 0l (page lsl t.pagesize) land ((1 lsl t.cachesize) - 1) in
   let offset = logical_address land ((t.pagesize lsl 1) - 1) in
@@ -507,12 +509,16 @@ let invalidate t ~off:logical_address ~len =
 
 let is_aligned x = x land ((1 lsl 2) - 1) == 0
 
+exception Out_of_bounds of int
+
+let[@inline never] out_of_bounds offset = raise (Out_of_bounds offset)
+
 let get_uint8 t logical_address =
   match load t ~len:1 logical_address with
   | Some { payload; _ } ->
       let offset = logical_address land ((1 lsl t.pagesize) - 1) in
       Bstr.get_uint8 payload offset
-  | None -> failwithf "Cachet.get_uint8"
+  | None -> out_of_bounds logical_address
 
 let get_int8 t logical_address =
   (get_uint8 t logical_address lsl (Sys.int_size - 8)) asr (Sys.int_size - 8)
@@ -523,7 +529,7 @@ let blit_to_bytes t ~src_off:logical_address buf ~dst_off ~len =
   let off = logical_address land ((1 lsl t.pagesize) - 1) in
   if is_aligned off && (1 lsl t.pagesize) - off >= len then begin
     match load t ~len logical_address with
-    | None -> failwithf "Cachet.blit_to_bytes"
+    | None -> out_of_bounds logical_address
     | Some slice ->
         Bstr.blit_to_bytes slice.payload ~src_off:off buf ~dst_off:0 ~len
   end
@@ -619,13 +625,14 @@ let iter_with_len t len ~fn logical_address =
           let logical_address = offset + (1 lsl t.pagesize) in
           match load t logical_address with
           | Some { payload; length; _ } ->
-              if len - max > length then failwith "Chat.iter_with_len";
+              if len - max > length then
+                out_of_bounds (logical_address + (len - max - 1));
               for i = 0 to len - max - 1 do
                 fn (Bstr.get_uint8 payload i)
               done
-          | None -> failwith "Chat.iter_with_len"
+          | None -> out_of_bounds logical_address
         end
-    | None -> failwith "Chat.iter_with_len"
+    | None -> out_of_bounds logical_address
   end
 
 let iter t ?len ~fn logical_address =
diff --git a/lib/cachet.mli b/lib/cachet.mli
index 764515b..e255021 100644
--- a/lib/cachet.mli
+++ b/lib/cachet.mli
@@ -281,13 +281,20 @@ val invalidate : 'fd t -> off:int -> len:int -> unit
     8-bit or 16-bit integers represented by [int] values sign-extend (resp.
     zero-extend) their result. *)
 
+exception Out_of_bounds of int
+(** If Cachet tries to retrieve a byte outside the block device, this exception is raised. *)
+
 val get_int8 : 'fd t -> int -> int
 (** [get_int8 t logical_address] is [t]'s signed 8-bit integer starting at byte
-    index [logical_address]. *)
+    index [logical_address].
+
+    @raise Out_of_bounds if [logical_address] is not accessible. *)
 
 val get_uint8 : 'fd t -> int -> int
 (** [get_uint8 t logical_address] is [t]'s unsigned 8-bit integer starting at
-    byte index [logical_address]. *)
+    byte index [logical_address].
+    
+    @raise Out_of_bounds if [logical_address] is not accessible. *)
 
 val get_uint16_ne : 'fd t -> int -> int
 val get_uint16_le : 'fd t -> int -> int
@@ -309,8 +316,8 @@ val get_string : 'fd t -> len:int -> int -> string
     You can use {!val:syscalls} to find out how many times [get_string] can
     call [map] at most.
 
-    @raise Failure if the [map] function cannot give us enough to copy [len]
-    bytes. *)
+    @raise Out_of_bounds if [logical_address] and [len] byte(s) are not
+    accessible. *)
 
 val get_seq : 'fd t -> int -> string Seq.t
 val next : 'fd t -> slice -> slice option