diff --git a/toonz/sources/tnztools/modifiers/modifiersegmentation.cpp b/toonz/sources/tnztools/modifiers/modifiersegmentation.cpp
index 1fbb4c0..ba1232e 100644
--- a/toonz/sources/tnztools/modifiers/modifiersegmentation.cpp
+++ b/toonz/sources/tnztools/modifiers/modifiersegmentation.cpp
@@ -72,7 +72,7 @@ TModifierSegmentation::modifyTrack(
   // remove points
   int start = track.size() - track.pointsAdded;
   if (start < 0) start = 0;
-  int subStart = subTrack.floorIndex(subTrack.indexByOriginalIndex(start-1)) + 1;
+  int subStart = start > 0 ? subTrack.floorIndex(subTrack.indexByOriginalIndex(start-1)) + 1 : 0;
   subTrack.truncate(subStart);
 
   // add points