diff --git a/aos/vision/blob/transpose.cc b/aos/vision/blob/transpose.cc
index 72e1cc1..fe4d4a9 100644
--- a/aos/vision/blob/transpose.cc
+++ b/aos/vision/blob/transpose.cc
@@ -61,7 +61,7 @@
       has_cur_range = true;
     };
 
-    int prev_start;
+    int prev_start = 0;
     for (const auto &pt : row_events) {
       switch (pt.second) {
         case kRangeStart: