diff --git a/src/unix/CCUnix.ml b/src/unix/CCUnix.ml
index 7a9e9e02..6b0c1bfb 100644
--- a/src/unix/CCUnix.ml
+++ b/src/unix/CCUnix.ml
@@ -34,10 +34,12 @@ let finally_ f x ~h =
     raise e
 
 (* print a string, but escaped if required *)
-let escape_str buf s =
-  if str_exists s
+let escape_str s =
+  if
+    str_exists s
       (function ' ' | '"' | '\'' | '\n' | '\t'-> true | _ -> false)
   then (
+    let buf = Buffer.create (String.length s) in
     Buffer.add_char buf '\'';
     String.iter
       (function
@@ -45,7 +47,8 @@ let escape_str buf s =
         | c -> Buffer.add_char buf c
       ) s;
     Buffer.add_char buf '\'';
-  ) else Buffer.add_string buf s
+    Buffer.contents buf
+  ) else s
 
 let read_all ?(size=1024) ic =
   let buf = ref (Bytes.create size) in
diff --git a/src/unix/CCUnix.mli b/src/unix/CCUnix.mli
index 8bcf017c..90a6e47d 100644
--- a/src/unix/CCUnix.mli
+++ b/src/unix/CCUnix.mli
@@ -13,13 +13,13 @@ type 'a gen = unit -> 'a option
 
 (** {2 Calling Commands} *)
 
-val escape_str : Buffer.t -> string -> unit
+val escape_str : string -> string
 (** Escape a string so it can be a shell argument. *)
 
 (*$T
-  CCPrint.sprintf "%a" escape_str "foo" = "foo"
-  CCPrint.sprintf "%a" escape_str "foo bar" = "'foo bar'"
-  CCPrint.sprintf "%a" escape_str "fo'o b'ar" = "'fo'\\''o b'\\''ar'"
+  escape_str "foo" = "foo"
+  escape_str "foo bar" = "'foo bar'"
+  escape_str "fo'o b'ar" = "'fo'\\''o b'\\''ar'"
 *)
 
 type call_result =
@@ -45,7 +45,7 @@ val call : ?bufsize:int ->
 
 (*$T
   (call ~stdin:(`Str "abc") "cat")#stdout = "abc"
-  (call "echo %a" escape_str "a'b'c")#stdout = "a'b'c\n"
+  (call "echo %s" (escape_str "a'b'c"))#stdout = "a'b'c\n"
   (call "echo %s" "a'b'c")#stdout = "abc\n"
 *)