diff --git a/library/gcm.c b/library/gcm.c
index 5121a7a..2ee5256 100644
--- a/library/gcm.c
+++ b/library/gcm.c
@@ -246,7 +246,7 @@
     for( i = 15; i >= 0; i-- )
     {
         lo = x[i] & 0xf;
-        hi = x[i] >> 4;
+        hi = ( x[i] >> 4 ) & 0xf;
 
         if( i != 15 )
         {