diff --git a/themes/brethil.zsh-theme b/themes/brethil.zsh-theme
index c0557e4..11445f8 100644
--- a/themes/brethil.zsh-theme
+++ b/themes/brethil.zsh-theme
@@ -128,13 +128,25 @@ _PROMPT_PROTO='$(virtualenv_info)$(user_prompt)$(path_prompt)$(git_prompt)$(job_
 
 function prompt_too_long(){
   local zero='%([BSUbfksu]|([FK]|){*})'
-  local stripped="$(echo ${(S%%)_PROMPT_PROTO//$~zero/} | sed "s,\x1B\[[0-9;]*[a-zA-Z],,g" ) "
+  if [[ -x $commands[gsed] ]]; then # fuck darwin
+    sed='gsed'
+  elif [[ "$(uname)" == "Darwin" ]]; then
+    local msg="error: multi-line prompt requires GNU sed"
+    msg+="install with \"brew install gnu-sed\""
+    echo "$msg"
+    return
+  else
+    sed='sed'
+  fi
+
+  local stripped="$(echo ${(S%%)_PROMPT_PROTO//$~zero/} | $sed 's|\x1B\[[0-9;]*[a-zA-Z]||g')"
   local prompt_len=${#stripped}
   local max_len=$(($COLUMNS/2))
   if [[ $prompt_len -ge $max_len ]]; then
     echo true
   fi
 }
+
 PROMPT="$(echo "$_PROMPT_PROTO")"
 NEWLINE=$'\n'
 PROMPT+='${$(prompt_too_long)/true/${NEWLINE}}'