diff -Nrup a/scripts/ruby/openbabel-ruby.cpp b/scripts/ruby/openbabel-ruby.cpp
--- a/scripts/ruby/openbabel-ruby.cpp	2016-10-10 17:27:32.000000000 +0200
+++ b/scripts/ruby/openbabel-ruby.cpp	2020-03-03 07:12:15.005496854 +0100
@@ -1260,7 +1260,7 @@ SWIGRUNTIME void SWIG_RubyInitializeTrac
     swig_ruby_trackings = (st_table*)NUM2SWIG(trackings_value);
   }
 
-  rb_define_virtual_variable("SWIG_TRACKINGS_COUNT", swig_ruby_trackings_count, NULL);
+  rb_define_virtual_variable("SWIG_TRACKINGS_COUNT", swig_ruby_trackings_count, (rb_gvar_setter_t *)NULL);
 }
 
 /* Add a Tracking from a C/C++ struct to a Ruby object */