diff --git a/jdmarker.c b/jdmarker.c
index 7332940..5837edb 100644
--- a/jdmarker.c
+++ b/jdmarker.c
@@ -1310,6 +1310,11 @@
   marker->pub.read_markers = read_markers;
   marker->pub.read_restart_marker = read_restart_marker;
   marker->pub.get_sos_marker_position = get_sos_marker_position;
+
+  // Initialize the SOS marker position to avoid underdefined behavior due to
+  // using a undefined field.
+  marker->pub.current_sos_marker_position = 0;
+
   /* Initialize COM/APPn processing.
    * By default, we examine and then discard APP0 and APP14,
    * but simply discard COM and all other APPn.