diff --git a/src/gray.f b/src/gray.f
index e9f9dd7..ac648db 100644
--- a/src/gray.f
+++ b/src/gray.f
@@ -3204,7 +3204,7 @@ c
         bdotgr=bdotgr+bv(iv)*dgr(iv)
         dbgr(iv)=0.0_wp_
         do jv=1,3
-          dbgr(iv)=dgr(jv)*derbv(jv,iv)+bv(jv)*ddgr(jv,iv)
+          dbgr(iv)=dbgr(iv)+dgr(jv)*derbv(jv,iv)+bv(jv)*ddgr(jv,iv)
         end do
       end do
 c