diff --git a/mrbgem.rake b/mrbgem.rake
index 9ae97b6..bc2d89a 100644
--- a/mrbgem.rake
+++ b/mrbgem.rake
@@ -5,4 +5,6 @@ MRuby::Gem::Specification.new('mruby-WiringPi') do |spec|
   spec.cc.include_paths << "#{spec.dir}/include" << "#{spec.dir}/wiringPi"
 
   spec.objs << (Dir.glob("#{dir}/src/*.c") + Dir.glob("#{dir}/wiringPi/*.c")).map { |f| f.relative_path_from(dir).pathmap("#{build_dir}/%X.o") }
+
+  spec.add_dependency('mruby-sprintf')
 end
diff --git a/src/wiringpi_wrap.c b/src/wiringpi_wrap.c
index 0f04ae3..4119d23 100644
--- a/src/wiringpi_wrap.c
+++ b/src/wiringpi_wrap.c
@@ -28,6 +28,8 @@
 #  define	FALSE	(1==2)
 #endif
 
+mrb_value mrb_str_format(mrb_state *, mrb_int, const mrb_value *, mrb_value);
+
 mrb_value
 mrb_WiringPi_Setup(mrb_state *mrb, mrb_value self)
 {