diff --git a/pngrutil.c b/pngrutil.c
index f6f98598f..e44c04e82 100644
--- a/pngrutil.c
+++ b/pngrutil.c
@@ -1,7 +1,7 @@
 
 /* pngrutil.c - utilities to read a PNG file
  *
- * Last changed in libpng 1.2.36 [May 7, 2009]
+ * Last changed in libpng 1.2.37 [May 13, 2009]
  * For conditions of distribution and use, see copyright notice in png.h
  * Copyright (c) 1998-2009 Glenn Randers-Pehrson
  * (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
@@ -3189,7 +3189,7 @@ defined(PNG_USER_TRANSFORM_PTR_SUPPORTED)
      png_free(png_ptr, png_ptr->big_row_buf);
       png_ptr->big_row_buf = (png_bytep)png_malloc(png_ptr, row_bytes + 64);
      if (png_ptr->interlaced)
-       png_memset(png_ptr->big_row_buf, 0, png_ptr->rowbytes + 64);
+       png_memset(png_ptr->big_row_buf, 0, row_bytes + 64);
      png_ptr->row_buf = png_ptr->big_row_buf + 32;
      png_ptr->old_big_row_buf_size = row_bytes + 64;
    }