diff --git a/src/lib/built_ins.ml b/src/lib/built_ins.ml
index 2f82878..6a2b5a3 100644
--- a/src/lib/built_ins.ml
+++ b/src/lib/built_ins.ml
@@ -2,6 +2,7 @@ module Options = struct
   type t = {
     noclobber : bool;
     pipefail : bool;
+    errexit : bool;
     no_path_expansion : bool;
     no_unset : bool;
     async : bool;
@@ -11,22 +12,25 @@ module Options = struct
     {
       noclobber = false;
       pipefail = false;
+      errexit = false;
       no_path_expansion = false;
       no_unset = false;
       async = false;
     }
 
-  let with_options ?noclobber ?pipefail ?async ?no_path_expansion ?no_unset t =
+  let with_options ?noclobber ?pipefail ?errexit ?async ?no_path_expansion
+      ?no_unset t =
     {
       noclobber = Option.value ~default:t.noclobber noclobber;
       pipefail = Option.value ~default:t.pipefail pipefail;
+      errexit = Option.value ~default:t.errexit errexit;
       async = Option.value ~default:t.async async;
       no_path_expansion =
         Option.value ~default:t.no_path_expansion no_path_expansion;
       no_unset = Option.value ~default:t.no_unset no_unset;
     }
 
-  type posix = [ `Noclobber | `Pipefail | `Noglob | `Nounset ]
+  type posix = [ `Noclobber | `Pipefail | `Noglob | `Nounset | `Errexit ]
   type merry = [ `Async ]
   type option = [ posix | merry ]
 
@@ -42,6 +46,7 @@ module Options = struct
         | `Pipefail -> with_options ~pipefail:true d
         | `Noclobber -> with_options ~noclobber:true d
         | `Noglob -> with_options ~no_path_expansion:true d
+        | `Errexit -> with_options ~errexit:true d
         | `Nounset -> with_options ~no_unset:true d
         | `Async -> with_options ~async:true d)
       t options
@@ -51,9 +56,12 @@ module Options = struct
       Fmt.pf ppf "%-12s %s@." name (if value then "on" else "off")
     in
     let opts =
-      let { noclobber; pipefail; async; no_path_expansion; no_unset } = opt in
+      let { noclobber; pipefail; async; no_path_expansion; no_unset; errexit } =
+        opt
+      in
       [
         ("pipefail", pipefail);
+        ("errexit", errexit);
         ("noclobber", noclobber);
         ("noglob", no_path_expansion);
         ("nounset", no_unset);
@@ -141,6 +149,7 @@ module Set = struct
     [
       ("pipefail", `Pipefail);
       ("noclobber", `Noclobber);
+      ("errexit", `Errexit);
       ("noglob", `Noglob);
       ("nounset", `Nounset);
       ("async", `Async);
@@ -150,6 +159,10 @@ module Set = struct
     let doc = "Options." in
     Arg.(value & opt_all (enum enum_map) [] & info [ "o" ] ~docv:"OPTION" ~doc)
 
+  let errexit =
+    let doc = "Exit on error, like -o errexit." in
+    Arg.(value & flag & info [ "e" ] ~docv:"ERREXIT" ~doc)
+
   let noclobber =
     let doc = "No clobber, like -o noclobber." in
     Arg.(value & flag & info [ "C" ] ~docv:"NOCLOBBER" ~doc)
@@ -163,14 +176,17 @@ module Set = struct
     Arg.(value & flag & info [ "u" ] ~docv:"NOUNSET" ~doc)
 
   let t =
-    let make_set update noglob noclobber nounset =
+    let make_set update noglob noclobber nounset errexit =
       let extra = if noglob then [ `Noglob ] else [] in
       let extra = if noclobber then `Noclobber :: extra else extra in
       let extra = if nounset then `Nounset :: extra else extra in
+      let extra = if errexit then `Errexit :: extra else extra in
       let update = extra @ update in
       Set { update; print_options = false }
     in
-    let term = Term.(const make_set $ option $ noglob $ noclobber $ nounset) in
+    let term =
+      Term.(const make_set $ option $ noglob $ noclobber $ nounset $ errexit)
+    in
     let info =
       let doc = "Set or unset options and positional parameters." in
       Cmd.info "set" ~doc
diff --git a/src/lib/built_ins.mli b/src/lib/built_ins.mli
index 2836b67..f8494f4 100644
--- a/src/lib/built_ins.mli
+++ b/src/lib/built_ins.mli
@@ -2,6 +2,7 @@ module Options : sig
   type t = {
     noclobber : bool;
     pipefail : bool;
+    errexit : bool;
     no_path_expansion : bool;
     no_unset : bool;
     async : bool;
@@ -9,7 +10,7 @@ module Options : sig
 
   val to_letters : t -> string
 
-  type posix = [ `Noclobber | `Pipefail | `Noglob | `Nounset ]
+  type posix = [ `Noclobber | `Pipefail | `Noglob | `Nounset | `Errexit ]
   type merry = [ `Async ]
   type option = [ posix | merry ]
 
@@ -18,6 +19,7 @@ module Options : sig
   val with_options :
     ?noclobber:bool ->
     ?pipefail:bool ->
+    ?errexit:bool ->
     ?async:bool ->
     ?no_path_expansion:bool ->
     ?no_unset:bool ->
diff --git a/test/simple.t b/test/simple.t
index 8051067..f1255c2 100644
--- a/test/simple.t
+++ b/test/simple.t
@@ -225,6 +225,17 @@ Clobbering files and stuff.
   msh: cannot overwrite existing file
   world
 
+Also errexit examples from the specification.
+
+  $ sh -c "set -e; (false; echo one) | cat; echo two"
+  two
+  $ msh -c "set -e; (false; echo one) | cat; echo two"
+  two
+  $ sh -c "set -e; echo $(false; echo one) two"
+  one two
+  $ msh -c "set -e; echo $(false; echo one) two"
+  one two
+
 2.7 Sequences
 
 A simple, semicolon sequence.
diff --git a/vendor/morbig.0.11.0/src/prelexerState.ml b/vendor/morbig.0.11.0/src/prelexerState.ml
index ac608eb..d967c9b 100644
--- a/vendor/morbig.0.11.0/src/prelexerState.ml
+++ b/vendor/morbig.0.11.0/src/prelexerState.ml
@@ -26,6 +26,12 @@ type atom =
 
 and quote_kind = SingleQuote | DoubleQuote | OpeningBrace
 
+let pp_atom ppf = function
+  | AssignmentMark -> Format.fprintf ppf "|=|"
+  | ArithmeticMark -> Format.fprintf ppf "|+|"
+  | QuotingMark _ -> Format.fprintf ppf "\""
+  | WordComponent (c, _) -> Format.fprintf ppf "%s" c 
+
 module AtomBuffer : sig
   type t
   val get : t -> atom list
@@ -317,7 +323,12 @@ let is_assignment_mark = function
 
 let recognize_assignment current =
   let rhs, prefix = take_until is_assignment_mark (buffer current) in
-  if prefix = buffer current then (
+  let is_empty_assignment = 
+    match rhs, prefix with
+    | [], AssignmentMark :: _ -> true
+    | _ -> false
+  in
+  if prefix = buffer current && not is_empty_assignment then (
     current
   ) else
     let buffer = AtomBuffer.make (rhs @ List.tl prefix) in