diff --git a/lib/KOReaderSync/ChapterXPathIndexer.h b/lib/KOReaderSync/ChapterXPathIndexer.h
index 356815f2..246fdecd 100644
--- a/lib/KOReaderSync/ChapterXPathIndexer.h
+++ b/lib/KOReaderSync/ChapterXPathIndexer.h
@@ -55,9 +55,13 @@ class ChapterXPathIndexer {
    * Parse DocFragment index from KOReader-style path segment:
    * /body/DocFragment[N]/body/...
    *
+   * KOReader uses 1-based DocFragment indices; N is converted to the 0-based
+   * spine index stored in outSpineIndex (i.e. outSpineIndex = N - 1).
+   *
    * @param xpath KOReader XPath
-   * @param outSpineIndex Parsed DocFragment index (0-based)
-   * @return true when DocFragment[N] exists and N is valid integer >= 0
+   * @param outSpineIndex 0-based spine index derived from DocFragment[N]
+   * @return true when DocFragment[N] exists and N is a valid integer >= 1
+   *         (converted to 0-based outSpineIndex); false otherwise
    */
   static bool tryExtractSpineIndexFromXPath(const std::string& xpath, int& outSpineIndex);
 };
diff --git a/lib/KOReaderSync/ProgressMapper.cpp b/lib/KOReaderSync/ProgressMapper.cpp
index b0199bf6..d62e0608 100644
--- a/lib/KOReaderSync/ProgressMapper.cpp
+++ b/lib/KOReaderSync/ProgressMapper.cpp
@@ -2,6 +2,7 @@
 
 #include <Logging.h>
 
+#include <algorithm>
 #include <cmath>
 
 #include "ChapterXPathIndexer.h"
@@ -71,7 +72,12 @@ CrossPointPosition ProgressMapper::toCrossPoint(const std::shared_ptr<Epub>& epu
       return result;
     }
 
-    const size_t targetBytes = static_cast<size_t>(bookSize * koPos.percentage);
+    if (!std::isfinite(koPos.percentage)) {
+      return result;
+    }
+
+    const float sanitizedPercentage = std::clamp(koPos.percentage, 0.0f, 1.0f);
+    const size_t targetBytes = static_cast<size_t>(bookSize * sanitizedPercentage);
 
     bool spineFound = false;
     for (int i = 0; i < spineCount; i++) {