diff --git a/IO/Image/vtkPNGWriter.cxx b/IO/Image/vtkPNGWriter.cxx
index 27df7f197294416b54a68bd4df5a1a796d60cd46..f47a487777705cccc8f3dd142ac16c79d8dbda56 100644
--- a/IO/Image/vtkPNGWriter.cxx
+++ b/IO/Image/vtkPNGWriter.cxx
@@ -220,7 +220,7 @@ void vtkPNGWriter::WriteSlice(vtkImageData *data, int* uExtent)
 
 
   this->TempFP = 0;
-  png_byte **row_pointers;
+  png_byte **row_pointers = 0;
   if (this->WriteToMemory)
     {
     vtkUnsignedCharArray *uc = this->GetResult();