diff --git a/cmake/ci.cmake b/cmake/ci.cmake
index 01ade8562..7d83aa3d6 100644
--- a/cmake/ci.cmake
+++ b/cmake/ci.cmake
@@ -359,7 +359,7 @@ set(GCC_CXXFLAGS "-std=c++11                          \
 add_custom_target(ci_test_gcc
     COMMAND CXX=${GCC_TOOL} CXXFLAGS=${GCC_CXXFLAGS} ${CMAKE_COMMAND}
         -DCMAKE_BUILD_TYPE=Debug -GNinja
-        -DJSON_BuildTests=ON
+        -DJSON_BuildTests=ON -DJSON_MultipleHeaders=ON
         -S${PROJECT_SOURCE_DIR} -B${PROJECT_BINARY_DIR}/build_gcc
     COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_gcc
     COMMAND cd ${PROJECT_BINARY_DIR}/build_gcc && ${CMAKE_CTEST_COMMAND} --parallel ${N} --output-on-failure
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 9a04768ca..8b3ede008 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -3940,7 +3940,7 @@ template<typename IteratorType> class iteration_proxy_value
     /// a string representation of the array index
     mutable string_type array_index_str = "0";
     /// an empty string (to return a reference for primitive values)
-    const string_type empty_str;
+    const string_type empty_str{};
 
   public:
     explicit iteration_proxy_value(IteratorType it) noexcept : anchor(it) {}