diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index dff84df3..86f73563 100644
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -357,8 +357,8 @@ namespace {
                 score += RookOnPawn * popcount(pos.pieces(Them, PAWN) & PseudoAttacks[ROOK][s]);
 
             // Bonus for rook on an open or semi-open file
-            if (pos.is_semiopen_file(Us, file_of(s)))
-                score += RookOnFile[bool(pos.is_semiopen_file(Them, file_of(s)))];
+            if (pos.is_on_semiopen_file(Us, s))
+                score += RookOnFile[bool(pos.is_on_semiopen_file(Them, s))];
 
             // Penalty when trapped by the king, even more if the king cannot castle
             else if (mob <= 3)
diff --git a/src/position.h b/src/position.h
index 5ff3d1ac..343751ed 100644
--- a/src/position.h
+++ b/src/position.h
@@ -96,7 +96,7 @@ public:
   template<PieceType Pt> int count() const;
   template<PieceType Pt> const Square* squares(Color c) const;
   template<PieceType Pt> Square square(Color c) const;
-  bool is_semiopen_file(Color c, File f) const;
+  bool is_on_semiopen_file(Color c, Square s) const;
 
   // Castling
   int castling_rights(Color c) const;
@@ -263,8 +263,8 @@ inline Square Position::ep_square() const {
   return st->epSquare;
 }
 
-inline bool Position::is_semiopen_file(Color c, File f) const {
-  return !(pieces(c, PAWN) & file_bb(f));
+inline bool Position::is_on_semiopen_file(Color c, Square s) const {
+  return !(pieces(c, PAWN) & file_bb(s));
 }
 
 inline bool Position::can_castle(CastlingRight cr) const {