diff --git a/Makefile b/Makefile
index 19cab9d..65bfec0 100644
--- a/Makefile
+++ b/Makefile
@@ -1,8 +1,8 @@
 ifeq ($(WARN),1)
 ifeq ($(CXX),g++)
-WARN_FLAGS = -O3 -g -Wall -Wextra -Werror # -Weffc++
+WARN_FLAGS = -O3 -g -Wall -Wextra -Wzero-as-null-pointer-constant -Werror # -Weffc++
 else ifeq ($(CXX),clang++)
-WARN_FLAGS = -O3 -g -Wall -Wextra -Werror
+WARN_FLAGS = -O3 -g -Wall -Wextra -Wzero-as-null-pointer-constant -Werror
 else ifeq ($(CXX),icpc)
 WARN_FLAGS = -O3 -ipo -g -Wall -wd981 -wd383 -wd2259 -Werror # -Weffc++
 endif
diff --git a/OptionParser.h b/OptionParser.h
index 1556091..1bab395 100644
--- a/OptionParser.h
+++ b/OptionParser.h
@@ -76,7 +76,7 @@ class Values {
 class Option {
   public:
     Option(const OptionParser& p) :
-      _parser(p), _action("store"), _type("string"), _nargs(1), _callback(0) {}
+      _parser(p), _action("store"), _type("string"), _nargs(1), _callback(nullptr) {}
     virtual ~Option() {}
 
     Option& action(const std::string& a);