diff --git a/pngtrans.c b/pngtrans.c
index c669217..191ec4d 100644
--- a/pngtrans.c
+++ b/pngtrans.c
@@ -609,7 +609,7 @@
       return; /* The filler channel has gone already */
 
    /* Fix the rowbytes value. */
-   row_info->rowbytes = (unsigned int)(dp-row);
+   row_info->rowbytes = (png_size_t)(dp-row);
 }
 #endif