Index: trunk/src/rotate.h
===================================================================
--- trunk/src/rotate.h	(revision 406)
+++ trunk/src/rotate.h	(revision 407)
@@ -59,16 +59,19 @@
 	int _n_; \
 	for(_n_ = n_in;_n_>0;_n_--) { \
 		int _old_, _nw_ = 0; \
-		_old_ = GET_SQUARE(p)-1; \
-		if (_old_ & 1) \
-			_nw_ |= 8; \
-		if (_old_ & 8) \
-			_nw_ |= 2; \
-		if (_old_ & 2) \
-			_nw_ |= 4; \
-		if (_old_ & 4) \
-			_nw_ |= 1; \
-		GET_SQUARE(p) = _nw_+1; \
+		_old_ = GET_SQUARE(p); \
+		if ((_old_ > 1) && (_old_ < 17)) { \
+			_old_--; \
+			if (_old_ & 1) \
+				_nw_ |= 8; \
+			if (_old_ & 8) \
+				_nw_ |= 2; \
+			if (_old_ & 2) \
+				_nw_ |= 4; \
+			if (_old_ & 4) \
+				_nw_ |= 1; \
+			GET_SQUARE(p) = _nw_+1; \
+		} \
 	} \
 } while(0)