Try to reproduce #691, no luck yet.

Author: cowtowncoder

src/test/java/com/fasterxml/jackson/databind/ser/TestMapSerialization.java

@@ -6,6 +6,9 @@
 
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonPropertyOrder;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import com.fasterxml.jackson.annotation.JsonTypeInfo.Id;
+import com.fasterxml.jackson.annotation.JsonTypeName;
 import com.fasterxml.jackson.core.*;
 import com.fasterxml.jackson.databind.*;
 import com.fasterxml.jackson.databind.annotation.JsonSerialize;
@@ -117,6 +120,11 @@ public NoEmptyStringsMap add(String key, String value) {
         }
     }
 
+    // for [databind#691]
+    @JsonTypeInfo(use=Id.NAME)
+    @JsonTypeName("mymap")
+    static class MapWithTypedValues extends LinkedHashMap<String,String> { }
+
     /*
     /**********************************************************
     /* Test methods
@@ -255,4 +263,17 @@ public void testClassKey() throws IOException
         String json = MAPPER.writeValueAsString(map);
         assertEquals(aposToQuotes("{'java.lang.String':2}"), json);
     }
+
+    // [databind#691]
+    public void testNullJsonMapping691() throws Exception
+    {
+        MapWithTypedValues input = new MapWithTypedValues();
+        input.put("id", "Test");
+        input.put("NULL", null);
+
+        String json = MAPPER.writeValueAsString(input);
+
+        assertEquals(aposToQuotes("{'@type':'mymap','id':'Test','NULL':null}"),
+                json);
+    }    
 }