diff --git a/lib/vf_common.cpp b/lib/vf_common.cpp
index eeb19c2381c..0498a7e303d 100644
--- a/lib/vf_common.cpp
+++ b/lib/vf_common.cpp
@@ -150,7 +150,7 @@ namespace ValueFlow
             if (!tok->isTemplateArg())
                 value.setKnown();
             setTokenValue(tok, std::move(value), settings);
-        } else if (tok->str() == "NULL" || (tok->isCpp() && tok->str() == "nullptr")) {
+        } else if (tok->str() == "NULL" || ((tok->isCpp() || settings.standards.c >= Standards::C23) && tok->str() == "nullptr")) {
             Value value(0);
             if (!tok->isTemplateArg())
                 value.setKnown();
diff --git a/test/testnullpointer.cpp b/test/testnullpointer.cpp
index 1e21479d5ab..1028c527275 100644
--- a/test/testnullpointer.cpp
+++ b/test/testnullpointer.cpp
@@ -183,12 +183,13 @@ class TestNullPointer : public TestFixture {
         CheckOptions() = default;
         bool inconclusive = false;
         bool cpp = true;
+        Standards::cstd_t cstd = Standards::CLatest;
     };
 
 #define check(...) check_(__FILE__, __LINE__, __VA_ARGS__)
     template<size_t size>
     void check_(const char* file, int line, const char (&code)[size], const CheckOptions& options = make_default_obj()) {
-        const Settings settings1 = settingsBuilder(settings).certainty(Certainty::inconclusive, options.inconclusive).build();
+        const Settings settings1 = settingsBuilder(settings).certainty(Certainty::inconclusive, options.inconclusive).c(options.cstd).build();
 
         // Tokenize..
         SimpleTokenizer tokenizer(settings1, *this, options.cpp);
@@ -1331,8 +1332,11 @@ class TestNullPointer : public TestFixture {
         check(code); // C++ file => nullptr means NULL
         ASSERT_EQUALS("[test.cpp:4:11]: (error) Null pointer dereference: i [nullPointer]\n", errout_str());
 
-        check(code, dinit(CheckOptions, $.cpp = false)); // C file => nullptr does not mean NULL
+        check(code, dinit(CheckOptions, $.cpp = false, $.cstd = Standards::C17)); // C17 file => nullptr does not mean NULL
         ASSERT_EQUALS("", errout_str());
+
+        check(code, dinit(CheckOptions, $.cpp = false));
+        ASSERT_EQUALS("[test.c:4:11]: (error) Null pointer dereference: i [nullPointer]\n", errout_str());
     }
 
     void nullpointer15() {  // #3560
diff --git a/test/testvalueflow.cpp b/test/testvalueflow.cpp
index d5a2b4f019b..86fa02b9191 100644
--- a/test/testvalueflow.cpp
+++ b/test/testvalueflow.cpp
@@ -5635,6 +5635,23 @@ class TestValueFlow : public TestFixture {
         value = valueOfTok(code, ", 1");
         ASSERT_EQUALS(0, value.intvalue);
         ASSERT_EQUALS(false, value.isKnown());
+
+        // #13959
+        const Settings settingsOld = settings;
+        settings = settingsBuilder(settingsOld).c(Standards::C23).build();
+        code = "void f(int* p) {\n"
+               "    if (p == nullptr)\n"
+               "        return;\n"
+               "    if (p) {}\n"
+               "}\n";
+        value = valueOfTok(code, "p ) { }", &settings, /*cpp*/ false);
+        ASSERT_EQUALS(1, value.intvalue);
+        ASSERT_EQUALS(true, value.isKnown());
+
+        settings = settingsBuilder(settingsOld).c(Standards::C17).build();
+        value = valueOfTok(code, "p ) { }", &settings, /*cpp*/ false);
+        ASSERT(value == ValueFlow::Value());
+        settings = settingsOld;
     }
 
     void valueFlowSizeofForwardDeclaredEnum() {