diff --git a/poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFSheet.java b/poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFSheet.java
index 4fc072ef951..128fcbd41ed 100644
--- a/poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFSheet.java
+++ b/poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFSheet.java
@@ -395,7 +395,11 @@ protected CTGroupShape getSpTree(){
             if(sp.length == 0) {
                 throw new IllegalStateException("CTGroupShape was not found");
             }
-            _spTree = (CTGroupShape)sp[0];
+            XmlObject xmlObject = sp[0];
+            if (!(xmlObject instanceof CTGroupShape)) {
+                throw new IllegalArgumentException("Had unexpected type of entry: " + xmlObject.getClass());
+            }
+            _spTree = (CTGroupShape) xmlObject;
         }
         return _spTree;
     }
diff --git a/poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFSlide.java b/poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFSlide.java
index c48c8d162f6..0b9e6a32129 100644
--- a/poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFSlide.java
+++ b/poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFSlide.java
@@ -271,9 +271,9 @@ public XSLFTheme getTheme(){
      */
     @Override
     public XSLFBackground getBackground() {
-        CTBackground bg = _slide.getCSld().getBg();
-        if(bg != null) {
-            return new XSLFBackground(bg, this);
+        if(_slide.getCSld() != null &&
+                _slide.getCSld().getBg() != null) {
+            return new XSLFBackground(_slide.getCSld().getBg(), this);
         } else {
             return getMasterSheet().getBackground();
         }
diff --git a/test-data/slideshow/clusterfuzz-testcase-minimized-POIXSLFFuzzer-5463285576892416.pptx b/test-data/slideshow/clusterfuzz-testcase-minimized-POIXSLFFuzzer-5463285576892416.pptx
new file mode 100644
index 00000000000..1038f440c2e
Binary files /dev/null and b/test-data/slideshow/clusterfuzz-testcase-minimized-POIXSLFFuzzer-5463285576892416.pptx differ
diff --git a/test-data/spreadsheet/stress.xls b/test-data/spreadsheet/stress.xls
index b797f92f285..75b297f57eb 100644
Binary files a/test-data/spreadsheet/stress.xls and b/test-data/spreadsheet/stress.xls differ