diff --git a/treap.cpp b/treap.cpp
index 705bf1c..4d0bc90 100644
--- a/treap.cpp
+++ b/treap.cpp
@@ -153,7 +153,7 @@ count_type Treap::get_tab_offset(position p) {
 	string line = get_line(p.r, false);
 	for(count_type i = 0; i + tab_offset < p.c; ++i) {
 		if(line[i] == '\t')
-			tab_offset += ((i+tab_offset-1) % TAB_SIZE);
+			tab_offset += TAB_SIZE - (i+tab_offset) % TAB_SIZE - 1;
 		if(i + tab_offset > p.c)
 			tab_offset -= i + tab_offset - p.c;
 	}