diff --git a/src/position.h b/src/position.h
index 85c4c29dc..a0da7de0c 100644
--- a/src/position.h
+++ b/src/position.h
@@ -671,11 +671,11 @@ inline Bitboard Position::drop_region(Color c, PieceType pt) const {
   {
       if (c == WHITE)
       {
-          b &= var->whitePieceDropRegion.boardOfPiece(toupper(piece_to_char()[(c << PIECE_TYPE_BITS) + pt]));
+          b &= var->whitePieceDropRegion.boardOfPiece(toupper(piece_to_char()[(c << PIECE_TYPE_BITS) | pt]));
       }
       else if (c == BLACK)
       {
-          b &= var->blackPieceDropRegion.boardOfPiece(toupper(piece_to_char()[(c << PIECE_TYPE_BITS) + pt]));
+          b &= var->blackPieceDropRegion.boardOfPiece(toupper(piece_to_char()[(c << PIECE_TYPE_BITS) | pt]));
       }
   }