diff --git a/commandline.cpp b/commandline.cpp
index f1efa5f5..f2d2bae3 100644
--- a/commandline.cpp
+++ b/commandline.cpp
@@ -100,10 +100,11 @@ EX namespace arg {
   EX bool argis(const string& s) { if(args()[0] == '-' && args()[1] == '-') return args().substr(1) == s; return args() == s; }
   
   EX void shift_arg_formula(ld& x, const reaction_t& r IS(reaction_t())) {
-    shift(); x = argf(); 
+    shift(); ld old = x; x = argf(); 
     #if CAP_ANIMATIONS
     anims::animate_parameter(x, args(), r); 
     #endif
+    if(old != x && r) r();
     }
   
   #if HDR