diff --git a/CMakeLists.txt b/CMakeLists.txt
index fcd7d505..a01886c4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -204,7 +204,7 @@ if((CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME) AND BUILD_TESTING)
                   -Wdouble-promotion
                   -Wno-sign-compare
                   -Wextra-semi
-                  #   -Wzero-as-null-pointer-constant
+                  -Wzero-as-null-pointer-constant
                   -Wundef)
     if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
         target_compile_options(htool INTERFACE -Wimplicit-fallthrough -Wextra-semi-stmt -ferror-limit=200)
diff --git a/include/htool/hmatrix/hmatrix_distributed_output.hpp b/include/htool/hmatrix/hmatrix_distributed_output.hpp
index 9ed73c51..ecc885a9 100644
--- a/include/htool/hmatrix/hmatrix_distributed_output.hpp
+++ b/include/htool/hmatrix/hmatrix_distributed_output.hpp
@@ -203,7 +203,7 @@ void print_distributed_hmatrix_information(const HMatrix<CoefficientPrecision, C
     if (rankWorld == 0) {
         std::size_t output_size = 2 + std::max_element(std::begin(distributed_information), std::end(distributed_information), [](const auto &a, const auto &b) { return a.first.size() < b.first.size(); })->first.size();
         // save default formatting
-        std::ios init(NULL);
+        std::ios init(nullptr);
         init.copyfmt(os);
 
         os << std::setfill('_') << std::left;
diff --git a/include/htool/hmatrix/hmatrix_output.hpp b/include/htool/hmatrix/hmatrix_output.hpp
index c1c528a2..d6b9c2a7 100644
--- a/include/htool/hmatrix/hmatrix_output.hpp
+++ b/include/htool/hmatrix/hmatrix_output.hpp
@@ -105,7 +105,7 @@ void print_tree_parameters(const HMatrix<CoefficientPrecision, CoordinatePrecisi
     int output_size      = 23;
 
     // save default formatting
-    std::ios init(NULL);
+    std::ios init(nullptr);
     init.copyfmt(os);
 
     os << std::setfill('_') << std::left;
@@ -227,7 +227,7 @@ void print_hmatrix_information(const HMatrix<CoefficientPrecision, CoordinatePre
     std::size_t output_size  = 2 + std::max_element(std::begin(hmatrix_information), std::end(hmatrix_information), [](const auto &a, const auto &b) { return a.first.size() < b.first.size(); })->first.size();
 
     // save default formatting
-    std::ios init(NULL);
+    std::ios init(nullptr);
     init.copyfmt(os);
 
     os << std::setfill('_') << std::left;