diff --git a/Storage/Disk/DiskImage/Formats/STX.cpp b/Storage/Disk/DiskImage/Formats/STX.cpp
index 95a965a51..27c15c49c 100644
--- a/Storage/Disk/DiskImage/Formats/STX.cpp
+++ b/Storage/Disk/DiskImage/Formats/STX.cpp
@@ -188,8 +188,10 @@ class TrackConstructor {
 			auto location = locations.begin();
 			track_position = track_data_.begin();
 			while(location != locations.end()) {
+				assert(location->position < track_data_.end());
+
 				// Advance to location.position.
-				while(track_position != location->position) {
+				while(track_position < location->position) {
 					encoder->add_byte(*track_position);
 					++track_position;
 				}