diff --git a/pngwio.c b/pngwio.c
index 8c1cab3..65ac856 100644
--- a/pngwio.c
+++ b/pngwio.c
@@ -34,7 +34,8 @@
 {
    /* NOTE: write_data_fn must not change the buffer! */
    if (png_ptr->write_data_fn != NULL )
-      (*(png_ptr->write_data_fn))(png_ptr, (png_bytep)data, length);
+      (*(png_ptr->write_data_fn))(png_ptr, png_constcast(png_bytep,data),
+         length);
 
    else
       png_error(png_ptr, "Call to NULL write function");