diff --git a/src/base/ftgrays.c b/src/base/ftgrays.c
index 0294fbfd4..151cceca9 100644
--- a/src/base/ftgrays.c
+++ b/src/base/ftgrays.c
@@ -1234,10 +1234,10 @@
         coverage = -coverage;
 
       while ( coverage >= 512 )
-        coverage = 512 - coverage;
+        coverage -= 512;
 
       if ( coverage > 256 )
-        coverage = 0;
+        coverage = 512 - coverage;
       else if ( coverage == 256 )
         coverage = 255;
     }