diff --git a/cfg/std.cfg b/cfg/std.cfg
index 6eaa28a05..35e4a0f1b 100644
--- a/cfg/std.cfg
+++ b/cfg/std.cfg
@@ -1480,9 +1480,6 @@
   <function name="strxfrm">
     <noreturn>false</noreturn>
     <leak-ignore/>
-    <arg nr="1">
-      <not-null/>
-    </arg>
     <arg nr="2">
       <not-null/>
       <not-uninit/>
diff --git a/test/testnullpointer.cpp b/test/testnullpointer.cpp
index 05b059a50..fc5a089c8 100644
--- a/test/testnullpointer.cpp
+++ b/test/testnullpointer.cpp
@@ -2575,6 +2575,13 @@ private:
 
         check("void f() { strtok(NULL, 'x');}");
         ASSERT_EQUALS("",errout.str());
+
+        // #6306 "false positive with strxfrm NULL argument"
+        check("void foo(void) { size_t res = strxfrm(NULL, \"foo\", 0); }");
+        ASSERT_EQUALS("",errout.str());
+        check("void foo(void) { size_t res = strxfrm(NULL, \"foo\", 42); }");
+        TODO_ASSERT_EQUALS("[test.cpp:1]: (error) Null pointer dereference\n", "", errout.str());
+
     }
 
     void nullpointerFputc() {