diff --git a/src/proj_transform.cpp b/src/proj_transform.cpp
index b312122c1..a6a010e3e 100644
--- a/src/proj_transform.cpp
+++ b/src/proj_transform.cpp
@@ -269,6 +269,11 @@ bool proj_transform::backward(box2d<double>& env, int points) const
     if (is_source_equal_dest_)
         return true;
 
+    if (wgs84_to_merc_ || merc_to_wgs84_)
+    {
+        return backward(env);
+    }
+
     std::vector<coord<double,2> > coords;
     envelope_points(coords, env, points);
 
@@ -294,6 +299,11 @@ bool proj_transform::forward(box2d<double>& env, int points) const
     if (is_source_equal_dest_)
         return true;
 
+    if (wgs84_to_merc_ || merc_to_wgs84_)
+    {
+        return forward(env);
+    }
+
     std::vector<coord<double,2> > coords;
     envelope_points(coords, env, points);