diff --git a/server/modules/filter/masking/maskingfiltersession.cc b/server/modules/filter/masking/maskingfiltersession.cc
index 6c86fda2f..6e7b32b6b 100644
--- a/server/modules/filter/masking/maskingfiltersession.cc
+++ b/server/modules/filter/masking/maskingfiltersession.cc
@@ -191,12 +191,6 @@ void MaskingFilterSession::handle_row(GWBUF* pPacket)
         m_state = EXPECTING_NOTHING;
         break;
 
-    case 0xfb: // NULL is sent as 0xfb
-        MXS_NOTICE("NULL");
-        // We must ask for the rule so as not to get out of sync.
-        m_res.get_rule();
-        break;
-
     default:
         {
             ComQueryResponse::Row row(response);
@@ -210,7 +204,10 @@ void MaskingFilterSession::handle_row(GWBUF* pPacket)
                 {
                     LEncString s = *i;
 
-                    pRule->rewrite(s);
+                    if (!s.is_null())
+                    {
+                        pRule->rewrite(s);
+                    }
 
                     MXS_NOTICE("String: %s", (*i).to_string().c_str());
                 }
diff --git a/server/modules/filter/masking/mysql.hh b/server/modules/filter/masking/mysql.hh
index 3f22a0798..56eba66c2 100644
--- a/server/modules/filter/masking/mysql.hh
+++ b/server/modules/filter/masking/mysql.hh
@@ -213,7 +213,16 @@ public:
      */
     LEncString(uint8_t* pData)
     {
-        m_pString = mxs_lestr_consume(&pData, &m_length);
+        // NULL is sent as 0xfb. See https://dev.mysql.com/doc/internals/en/com-query-response.html
+        if (*pData != 0xfb)
+        {
+            m_pString = mxs_lestr_consume(&pData, &m_length);
+        }
+        else
+        {
+            m_pString = NULL;
+            m_length = 0;
+        }
     }
 
     /**
@@ -225,7 +234,17 @@ public:
      */
     LEncString(uint8_t** ppData)
     {
-        m_pString = mxs_lestr_consume(ppData, &m_length);
+        // NULL is sent as 0xfb. See https://dev.mysql.com/doc/internals/en/com-query-response.html
+        if (**ppData != 0xfb)
+        {
+            m_pString = mxs_lestr_consume(ppData, &m_length);
+        }
+        else
+        {
+            m_pString = NULL;
+            m_length = 0;
+            ++(*ppData);
+        }
     }
 
     /**
@@ -301,7 +320,14 @@ public:
      */
     std::string to_string() const
     {
-        return std::string(m_pString, m_length);
+        if (m_pString)
+        {
+            return std::string(m_pString, m_length);
+        }
+        else
+        {
+            return std::string("NULL");
+        }
     }
 
     /**
@@ -317,6 +343,16 @@ public:
         return o;
     }
 
+    /**
+     * Is NULL
+     *
+     * @return True, if the string represents a NULL value.
+     */
+    bool is_null() const
+    {
+        return m_pString == NULL;
+    }
+
 private:
     char*  m_pString; /*<! Pointer to beginning of string, NOT zero-terminated. */
     size_t m_length;  /*<! Length of string. */