diff --git a/src/line_gap.sml b/src/line_gap.sml
index 9de2d5e..253c913 100644
--- a/src/line_gap.sml
+++ b/src/line_gap.sml
@@ -3308,8 +3308,16 @@ struct
           val prevIdx = curIdx - String.size shd
         in
           if findLine >= prevLine then
-            let val relativeLine = findLine - prevLine - 1
-            in Vector.sub (lhd, relativeLine) + prevIdx
+            let
+              val relativeLine = findLine - prevLine - 1
+            in
+              if relativeLine < 0 then
+                (* the line start is not at this node,
+                 * so we need to continue going left
+                 * to find what index the line start corresponds to *)
+                lineNumberToIdxLeft (findLine, prevIdx, prevLine, stl, ltl)
+              else
+                Vector.sub (lhd, relativeLine) + prevIdx
             end
           else
             lineNumberToIdxLeft (findLine, prevIdx, prevLine, stl, ltl)