diff --git a/datatypes/mcs_datatype.h b/datatypes/mcs_datatype.h
index 93b673070..7ab9e5fac 100644
--- a/datatypes/mcs_datatype.h
+++ b/datatypes/mcs_datatype.h
@@ -490,7 +490,7 @@ inline bool isUnsignedInteger(const datatypes::SystemCatalog::ColDataType type)
  */
 inline bool isUnsigned(const datatypes::SystemCatalog::ColDataType type)
 {
-  if (isUnsignedInteger(type)) 
+  if (isUnsignedInteger(type))
     return true;
 
   switch (type)
@@ -518,14 +518,18 @@ inline bool isSignedInteger(const datatypes::SystemCatalog::ColDataType type)
   }
 }
 
-inline bool sameSignednessInteger(const datatypes::SystemCatalog::ColDataType type1, const datatypes::SystemCatalog::ColDataType type2)
+inline bool sameSignednessInteger(const datatypes::SystemCatalog::ColDataType type1,
+                                  const datatypes::SystemCatalog::ColDataType type2)
 {
-  return (isSignedInteger(type1) && isSignedInteger(type2)) || (isUnsignedInteger(type1) && isUnsignedInteger(type2));
+  return (isSignedInteger(type1) && isSignedInteger(type2)) ||
+         (isUnsignedInteger(type1) && isUnsignedInteger(type2));
 }
 
-inline bool differentSignednessInteger(const datatypes::SystemCatalog::ColDataType type1, const datatypes::SystemCatalog::ColDataType type2)
+inline bool differentSignednessInteger(const datatypes::SystemCatalog::ColDataType type1,
+                                       const datatypes::SystemCatalog::ColDataType type2)
 {
-  return (isSignedInteger(type1) && isUnsignedInteger(type2)) || (isUnsignedInteger(type1) && isSignedInteger(type2));
+  return (isSignedInteger(type1) && isUnsignedInteger(type2)) ||
+         (isUnsignedInteger(type1) && isSignedInteger(type2));
 }
 
 inline void promoteSignedInteger(datatypes::SystemCatalog::TypeHolderStd& unionedType)
@@ -568,7 +572,7 @@ inline void promoteSignedInteger(datatypes::SystemCatalog::TypeHolderStd& unione
       unionedType.precision = datatypes::INT128MAXPRECISION;
       unionedType.scale = 0;
       return;
-    } 
+    }
 
     default: idbassert(0); throw std::logic_error("datatypes::upcastSignedInteger: bad data type");
   }
@@ -590,40 +594,6 @@ namespace datatypes
 static constexpr int128_t minInt128 = int128_t(0x8000000000000000LL) << 64;
 static constexpr int128_t maxInt128 = (int128_t(0x7FFFFFFFFFFFFFFFLL) << 64) + 0xFFFFFFFFFFFFFFFFLL;
 
-class ConstString
-{
-  const char* m_str;
-  size_t m_length;
-
- public:
-  ConstString(const char* str, size_t length) : m_str(str), m_length(length)
-  {
-  }
-  const char* str() const
-  {
-    return m_str;
-  }
-  const char* end() const
-  {
-    return m_str + m_length;
-  }
-  size_t length() const
-  {
-    return m_length;
-  }
-  void bin2hex(char* o)
-  {
-    static const char hexdig[] = {'0', '1', '2', '3', '4', '5', '6', '7',
-                                  '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};
-    const char* e = end();
-    for (const char* s = m_str; s < e; s++)
-    {
-      *o++ = hexdig[*s >> 4];
-      *o++ = hexdig[*s & 0xf];
-    }
-  }
-};
-
 enum class round_style_t : uint8_t
 {
   NONE = 0,
diff --git a/dbcon/mysql/ha_mcs_datatype.h b/dbcon/mysql/ha_mcs_datatype.h
index 1ffafc38d..dbbbbe1d5 100644
--- a/dbcon/mysql/ha_mcs_datatype.h
+++ b/dbcon/mysql/ha_mcs_datatype.h
@@ -92,7 +92,7 @@ class StoreFieldMariaDB : public StoreField
     {
       size_t ll = length * 2;
       boost::scoped_array<char> sca(new char[ll]);
-      ConstString(str, length).bin2hex(sca.get());
+      utils::ConstString(str, length).bin2hex(sca.get());
       return m_field->store_binary(sca.get(), ll);
     }
     return m_field->store_binary(str, length);
@@ -103,7 +103,7 @@ class StoreFieldMariaDB : public StoreField
     return m_field->store(val, 0);
   }
 
-  int store_ulonglong(uint64_t val)override
+  int store_ulonglong(uint64_t val) override
   {
     return m_field->store(static_cast<int64_t>(val), 1);
   }
@@ -844,4 +844,3 @@ class WriteBatchFieldMariaDB : public WriteBatchField
 };
 
 }  // end of namespace datatypes
-
diff --git a/utils/common/conststring.h b/utils/common/conststring.h
index c1024c0a7..d29ac76b0 100644
--- a/utils/common/conststring.h
+++ b/utils/common/conststring.h
@@ -19,18 +19,21 @@
 
 #include <string>
 #include <string.h>
+#include <execinfo.h>
+#include "exceptclasses.h"
 
 namespace utils
 {
 class ConstString
 {
  protected:
-  const char* mStr;
+  const char* mStr;  // it can be NULL now.
   size_t mLength;
 
  public:
   ConstString(const char* str, size_t length) : mStr(str), mLength(length)
   {
+    idbassert(mStr || mLength == 0);  // nullptr mStr should have zero length.
   }
   explicit ConstString(const std::string& str) : mStr(str.data()), mLength(str.length())
   {
@@ -49,6 +52,7 @@ class ConstString
   }
   std::string toString() const
   {
+    idbassert(mStr);
     return std::string(mStr, mLength);
   }
   bool eq(char ch) const
@@ -73,6 +77,21 @@ class ConstString
     }
     return *this;
   }
+  void bin2hex(char* o)
+  {
+    static const char hexdig[] = {'0', '1', '2', '3', '4', '5', '6', '7',
+                                  '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};
+    const char* e = end();
+    for (const char* s = mStr; s < e; s++)
+    {
+      *o++ = hexdig[*s >> 4];
+      *o++ = hexdig[*s & 0xf];
+    }
+  }
+  bool isNull() const
+  {
+    return mStr == nullptr;
+  }
 };
 
 }  // namespace utils