diff --git a/src/ruby/ext/grpc/extconf.rb b/src/ruby/ext/grpc/extconf.rb
index de5b0ea3362a13a718e310756188d652c8adf225..f49fbdecbc946c2c83b917fc7b33e36f3268672f 100644
--- a/src/ruby/ext/grpc/extconf.rb
+++ b/src/ruby/ext/grpc/extconf.rb
@@ -85,7 +85,7 @@ else
     end
     unless File.exist?(File.join(grpc_lib_dir, 'libgrpc.a'))
       print "Building internal gRPC\n"
-      system("make -C #{root} static_c CONFIG=#{config}")
+      system("make -C #{grpc_root} static_c CONFIG=#{grpc_config}")
     end
     $CFLAGS << ' -I' + File.join(grpc_root, 'include')
     $LDFLAGS << ' -L' + grpc_lib_dir