diff --git a/gui.h b/gui.h
index 641d039..93a6937 100644
--- a/gui.h
+++ b/gui.h
@@ -59,7 +59,7 @@ public:
 	double perpendicular(position_f_2d &pos, position_f_2d &base) {
 		position_f_2d p = pos - base;
 		double l = p.length();
-		return l == 0.0 ? 0.0 : l = sinalpha(p);
+		return l == 0.0 ? 0.0 : l * sinalpha(p);
 	}
 
 	double length() { return hypot(x, y); }