Index: src/natools-chunked_strings.adb
==================================================================
--- src/natools-chunked_strings.adb
+++ src/natools-chunked_strings.adb
@@ -1452,11 +1452,15 @@
 
       case (Going) is
       when Ada.Strings.Forward =>
          loop
             Result := Ada.Strings.Fixed.Index
-              (Source.Data (Chunk).all, Set, Offset, Test, Going);
+              (Source.Data (Chunk).all
+                 (1 .. Positive'Min (Source.Size
+                                       - (Chunk - 1) * Source.Chunk_Size,
+                                     Source.Chunk_Size)),
+               Set, Offset, Test, Going);
             if Result /= 0 then
                return (Chunk - 1) * Source.Chunk_Size + Result;
             end if;
             if Chunk = Source.Data'Last then
                return 0;
@@ -1465,11 +1469,15 @@
             Offset := 1;
          end loop;
       when Ada.Strings.Backward =>
          loop
             Result := Ada.Strings.Fixed.Index
-              (Source.Data (Chunk).all, Set, Offset, Test, Going);
+              (Source.Data (Chunk).all
+                 (1 .. Positive'Min (Source.Size
+                                       - (Chunk - 1) * Source.Chunk_Size,
+                                     Source.Chunk_Size)),
+               Set, Offset, Test, Going);
             if Result /= 0 then
                return (Chunk - 1) * Source.Chunk_Size + Result;
             end if;
             if Chunk = Source.Data'First then
                return 0;