diff --git a/viewer/JFJochHttpReader.cpp b/viewer/JFJochHttpReader.cpp
index 67d671e4..4385297d 100644
--- a/viewer/JFJochHttpReader.cpp
+++ b/viewer/JFJochHttpReader.cpp
@@ -298,6 +298,10 @@ std::shared_ptr<JFJochReaderDataset> JFJochHttpReader::UpdateDataset_i() {
 void JFJochHttpReader::ReadURL(const std::string &url) {
     std::unique_lock ul(http_mutex);
     ResetConnection();  // selecting an address (re)opens the connection from scratch
+    // Drop the previous source's cached mask: a different (or re-armed) source has its own mask, and
+    // the arm_date key alone does not distinguish an empty/duplicate arm_date across sources.
+    cached_pixel_mask.reset();
+    cached_pixel_mask_arm_date.clear();
     addr = url;
     if (url.empty())
         SetStartMessage({});