diff --git a/deps/agg/include/agg_color_gray.h b/deps/agg/include/agg_color_gray.h
index f2cd0ff9c..bf300b8f1 100644
--- a/deps/agg/include/agg_color_gray.h
+++ b/deps/agg/include/agg_color_gray.h
@@ -652,8 +652,8 @@ struct gray16
     //--------------------------------------------------------------------
     self_type& opacity(double a_)
     {
-        if (a_ < 0) a_ = 0;
-        else if(a_ > 1) a_ = 1;
+        if (a_ < 0) a = 0;
+        else if(a_ > 1) a = 1;
         else a = (value_type)uround(a_ * double(base_mask));
         return *this;
     }