diff -r 2aed5418e142 -r 6008840b859e src/rules/bishop.c
--- a/src/rules/bishop.c	Sat Mar 29 14:46:33 2014 +0100
+++ b/src/rules/bishop.c	Sat Mar 29 16:53:58 2014 +0100
@@ -109,7 +109,7 @@
                 move->fromfile = file;
             }
             file = move->tofile - d;
-            if (isfile(file) && board[row][file] == move->piece) {
+            if (isidx(file) && board[row][file] == move->piece) {
                 if (amb) {
                     return AMBIGUOUS_MOVE;
                 }