diff --git a/src/threads/CCSemaphore.ml b/src/threads/CCSemaphore.ml
index 22673dfd..17d0b6de 100644
--- a/src/threads/CCSemaphore.ml
+++ b/src/threads/CCSemaphore.ml
@@ -6,11 +6,12 @@ type t = {
   cond : Condition.t;
 }
 
-let create n = {
-  n;
-  mutex=Mutex.create();
-  cond=Condition.create();
-}
+let create n =
+  if n <= 0 then invalid_arg "Semaphore.create";
+  { n;
+    mutex=Mutex.create();
+    cond=Condition.create();
+  }
 
 let get t = t.n
 
diff --git a/src/threads/CCSemaphore.mli b/src/threads/CCSemaphore.mli
index 7f8c9ad6..5734d31c 100644
--- a/src/threads/CCSemaphore.mli
+++ b/src/threads/CCSemaphore.mli
@@ -9,13 +9,13 @@ type t
 
 val create : int -> t
 (** [create n] creates a semaphore with initial value [n]
-    @raise Invalid_argument if [n < 0] *)
+    @raise Invalid_argument if [n <= 0] *)
 
 val get : t -> int
 (** Current value *)
 
 val acquire : int -> t -> unit
-(** [acquire n s] blocks until [get s > n], then atomically
+(** [acquire n s] blocks until [get s >= n], then atomically
     sets [s := !s - n] *)
 
 val release : int -> t -> unit