diff --git a/poi-ooxml/src/main/java/org/apache/poi/xssf/eventusermodel/XSSFReader.java b/poi-ooxml/src/main/java/org/apache/poi/xssf/eventusermodel/XSSFReader.java
index 55fe271c79..8bd52bb9e8 100644
--- a/poi-ooxml/src/main/java/org/apache/poi/xssf/eventusermodel/XSSFReader.java
+++ b/poi-ooxml/src/main/java/org/apache/poi/xssf/eventusermodel/XSSFReader.java
@@ -380,6 +380,10 @@ public class XSSFReader {
          */
         @Override
         public InputStream next() {
+            if (!sheetIterator.hasNext()) {
+                throw new IllegalStateException("Cannot get next from iterator");
+            }
+
             xssfSheetRef = sheetIterator.next();
 
             String sheetId = xssfSheetRef.getId();
diff --git a/poi-ooxml/src/main/java/org/apache/poi/xssf/usermodel/XSSFWorkbook.java b/poi-ooxml/src/main/java/org/apache/poi/xssf/usermodel/XSSFWorkbook.java
index c48c338e6d..e1005ee1a4 100644
--- a/poi-ooxml/src/main/java/org/apache/poi/xssf/usermodel/XSSFWorkbook.java
+++ b/poi-ooxml/src/main/java/org/apache/poi/xssf/usermodel/XSSFWorkbook.java
@@ -831,7 +831,7 @@ public class XSSFWorkbook extends POIXMLDocument implements Workbook, Date1904Su
     private XSSFName createAndStoreName(CTDefinedName ctName) {
         XSSFName name = new XSSFName(ctName, this);
         namedRanges.add(name);
-        namedRangesByName.put(ctName.getName().toLowerCase(Locale.ENGLISH), name);
+        namedRangesByName.put(ctName.getName() == null ? null : ctName.getName().toLowerCase(Locale.ENGLISH), name);
         return name;
     }
 
diff --git a/test-data/spreadsheet/clusterfuzz-testcase-minimized-POIXSSFFuzzer-6123461607817216.xlsx b/test-data/spreadsheet/clusterfuzz-testcase-minimized-POIXSSFFuzzer-6123461607817216.xlsx
new file mode 100644
index 0000000000..927acf2e94
Binary files /dev/null and b/test-data/spreadsheet/clusterfuzz-testcase-minimized-POIXSSFFuzzer-6123461607817216.xlsx differ
diff --git a/test-data/spreadsheet/stress.xls b/test-data/spreadsheet/stress.xls
index 5af5cc74d0..fd160eeb72 100644
Binary files a/test-data/spreadsheet/stress.xls and b/test-data/spreadsheet/stress.xls differ