1 files changed, 4 insertions, 4 deletions
diff --git a/stdlib/string.ml b/stdlib/string.ml
index 2cdb83b34..6f2495c7e 100644
--- a/stdlib/string.ml
+++ b/stdlib/string.ml
@@ -38,7 +38,7 @@ let copy s =
   r
 
 let sub s ofs len =
-  if ofs < 0 || len < 0 || ofs + len > length s
+  if ofs < 0 || len < 0 || ofs > length s - len
   then invalid_arg "String.sub"
   else begin
     let r = create len in
@@ -47,13 +47,13 @@ let sub s ofs len =
   end
 
 let fill s ofs len c =
-  if ofs < 0 || len < 0 || ofs + len > length s
+  if ofs < 0 || len < 0 || ofs > length s - len
   then invalid_arg "String.fill"
   else unsafe_fill s ofs len c
 
 let blit s1 ofs1 s2 ofs2 len =
-  if len < 0 || ofs1 < 0 || ofs1 + len > length s1
-             || ofs2 < 0 || ofs2 + len > length s2
+  if len < 0 || ofs1 < 0 || ofs1 > length s1 - len
+             || ofs2 < 0 || ofs2 > length s2 - len
   then invalid_arg "String.blit"
   else unsafe_blit s1 ofs1 s2 ofs2 len