diff --git a/cfg/std.cfg b/cfg/std.cfg
index 098f6aa02..b30583835 100644
--- a/cfg/std.cfg
+++ b/cfg/std.cfg
@@ -197,14 +197,14 @@
   <function name="mbstowcs">
         <noreturn>false</noreturn>
         <leak-ignore/>
-        <arg nr="1"><not-null/></arg>
+        <arg nr="1"><not-uninit/></arg>
         <arg nr="2"><not-null/><not-uninit/></arg>
         <arg nr="3"><not-uninit/></arg>
   </function>
   <function name="wcstombs">
         <noreturn>false</noreturn>
         <leak-ignore/>
-        <arg nr="1"><not-null/></arg>
+        <arg nr="1"><not-uninit/></arg>
         <arg nr="2"><not-null/><not-uninit/></arg>
         <arg nr="3"><not-uninit/></arg>
   </function>
diff --git a/test/testnullpointer.cpp b/test/testnullpointer.cpp
index 8aea7490f..c4b45f59f 100644
--- a/test/testnullpointer.cpp
+++ b/test/testnullpointer.cpp
@@ -2562,6 +2562,12 @@ private:
 
         check("void f(char * p,char * q){ strtol (p,q,0);if(!p){}}");
         ASSERT_EQUALS(errp,errout.str());
+
+        // #6100 False positive nullPointer - calling mbstowcs(NULL,)
+        check("size_t get (char *value) { return mbstowcs (NULL, value, 0); }");
+        ASSERT_EQUALS("",errout.str());
+        check("size_t get (wchar_t *value) { return wcstombs (NULL, value, 0); }");
+        ASSERT_EQUALS("",errout.str());
     }
 
     void nullpointerFputc() {