diff --git a/src/osgPlugins/pnm/ReaderWriterPNM.cpp b/src/osgPlugins/pnm/ReaderWriterPNM.cpp
index 66e51c36c..1dac0365b 100644
--- a/src/osgPlugins/pnm/ReaderWriterPNM.cpp
+++ b/src/osgPlugins/pnm/ReaderWriterPNM.cpp
@@ -468,8 +468,9 @@ class ReaderWriterPNM : public osgDB::ReaderWriter
 
         virtual WriteResult writeImage(const osg::Image& image,const std::string& fileName, const osgDB::ReaderWriter::Options* options) const
         {
+            // Only ppm format output supported
             std::string ext = osgDB::getFileExtension(fileName);
-            if (!acceptsExtension("ppm")) return WriteResult::FILE_NOT_HANDLED;
+            if ( !osgDB::equalCaseInsensitive(ext, "ppm") ) return WriteResult::FILE_NOT_HANDLED;
             
             // only support rgb images right now.
             if (image.getPixelFormat()!=GL_RGB || image.getDataType()!=GL_UNSIGNED_BYTE) return WriteResult("Error image pixel format not supported by pnm writer.");