diff --git a/oox/source/export/ThemeExport.cxx b/oox/source/export/ThemeExport.cxx
index bdd79016aac7..986f804df6e1 100644
--- a/oox/source/export/ThemeExport.cxx
+++ b/oox/source/export/ThemeExport.cxx
@@ -903,6 +903,7 @@ bool ThemeExport::writeColorSet(model::Theme const& rTheme)
 
                 // drawingML alpha is a percentage on a 0..100000 scale.
                 sal_Int32 nAlpha = aColor.GetAlpha() * oox::drawingml::MAX_PERCENT / 255;
+                assert(nAlpha >= 0);
                 mpFS->singleElementNS(XML_a, XML_alpha, XML_val, OUString::number(nAlpha));
 
                 mpFS->endElementNS(XML_a, XML_srgbClr);
diff --git a/oox/source/export/drawingml.cxx b/oox/source/export/drawingml.cxx
index 256e7ecdf600..e6608a4fb4dc 100644
--- a/oox/source/export/drawingml.cxx
+++ b/oox/source/export/drawingml.cxx
@@ -363,6 +363,7 @@ void DrawingML::WriteColor( ::Color nColor, sal_Int32 nAlpha )
     if( nAlpha < MAX_PERCENT )
     {
         mpFS->startElementNS(XML_a, XML_srgbClr, XML_val, sColor);
+        assert(nAlpha >= 0);
         mpFS->singleElementNS(XML_a, XML_alpha, XML_val, OString::number(nAlpha));
         mpFS->endElementNS( XML_a, XML_srgbClr );
 
@@ -409,6 +410,7 @@ void DrawingML::WriteColor( const ::Color nColor, const Sequence< PropertyValue
     else if(nAlpha < MAX_PERCENT)
     {
         mpFS->startElementNS(XML_a, XML_srgbClr, XML_val, sColor);
+        assert(nAlpha >= 0);
         mpFS->singleElementNS(XML_a, XML_alpha, XML_val, OString::number(nAlpha));
         mpFS->endElementNS(XML_a, XML_srgbClr);
     }
@@ -427,6 +429,7 @@ void DrawingML::WriteColorTransformations( const Sequence< PropertyValue >& aTra
         {
             if(nToken == XML_alpha && nAlpha < MAX_PERCENT)
             {
+                assert(nAlpha >= 0);
                 mpFS->singleElementNS(XML_a, nToken, XML_val, OString::number(nAlpha));
             }
             else
diff --git a/xmloff/source/style/xmlbahdl.cxx b/xmloff/source/style/xmlbahdl.cxx
index a59abbdeed04..2749cff5356b 100644
--- a/xmloff/source/style/xmlbahdl.cxx
+++ b/xmloff/source/style/xmlbahdl.cxx
@@ -455,8 +455,14 @@ bool XMLNegPercentPropHdl::importXML( const OUString& rStrImpValue, Any& rValue,
 {
     sal_Int32 nValue = 0;
     bool bRet = ::sax::Converter::convertPercent( nValue, rStrImpValue );
-    if (bRet)
-        bRet = !o3tl::checked_sub<sal_Int32>(100, nValue, nValue);
+    if (!bRet)
+        return false;
+    if (nValue < 0 || nValue > 100)
+    {
+        SAL_WARN("xmloff", "Percentage property out of range, ignoring");
+        return false;
+    }
+    bRet = !o3tl::checked_sub<sal_Int32>(100, nValue, nValue);
     if (bRet)
         lcl_xmloff_setAny( rValue, nValue, nBytes );
     return bRet;