diff --git a/libImaging/AlphaComposite.c b/libImaging/AlphaComposite.c
index 93afe2a95..9433fae53 100644
--- a/libImaging/AlphaComposite.c
+++ b/libImaging/AlphaComposite.c
@@ -69,21 +69,22 @@ ImagingAlphaComposite(Imaging imDst, Imaging imSrc)
                 // almost equivalent to:
                 // tmp = a + (2 << (n-1)), ((tmp >> n) + tmp) >> n
 
+                UINT32 tmpr, tmpg, tmpb;
                 UINT16 blend = dst->a * (255 - src->a);
                 UINT16 outa255 = src->a * 255 + blend;
                 // There we use 7 bits for precision.
                 // We could use more, but we go beyond 32 bits.
                 UINT16 coef1 = src->a * 255 * 255 * 128 / outa255;
-                UINT16 coef2 = blend * 255 * 128 / outa255;
+                UINT16 coef2 = 255 * 128 - coef1;
 
                 #define SHIFTFORDIV255(a)\
-                    ((a >> 8) + a >> 8)
+                    ((((a) >> 8) + a) >> 8)
 
-                UINT32 tmpr = src->r * coef1 + dst->r * coef2 + (0x80 << 7);
+                tmpr = src->r * coef1 + dst->r * coef2 + (0x80 << 7);
                 out->r = SHIFTFORDIV255(tmpr) >> 7;
-                UINT32 tmpg = src->g * coef1 + dst->g * coef2 + (0x80 << 7);
+                tmpg = src->g * coef1 + dst->g * coef2 + (0x80 << 7);
                 out->g = SHIFTFORDIV255(tmpg) >> 7;
-                UINT32 tmpb = src->b * coef1 + dst->b * coef2 + (0x80 << 7);
+                tmpb = src->b * coef1 + dst->b * coef2 + (0x80 << 7);
                 out->b = SHIFTFORDIV255(tmpb) >> 7;
                 out->a = SHIFTFORDIV255(outa255 + 0x80);
             }