diff --git a/pngwrite.c b/pngwrite.c
index e526cba..b91d04d 100644
--- a/pngwrite.c
+++ b/pngwrite.c
@@ -1636,7 +1636,7 @@
  * calculation can be done to 15 bits of accuracy; however, the output needs to
  * be scaled in the range 0..255*65535, so include that scaling here.
  */
-#   define UNP_RECIPROCAL(alpha) ((((0xffff*0xff)<<7)+(alpha>>1))/alpha)
+#   define UNP_RECIPROCAL(alpha) ((((0xffff*0xff)<<7)+((alpha)>>1))/(alpha))
 
 static png_byte
 png_unpremultiply(png_uint_32 component, png_uint_32 alpha,