code: private TypeSerializer<Object> createSerializer(Object key, int pos) {
  		if (key == null) {
  			throw new NullKeyFieldException(pos);
  		}
  		try {
  			TypeInformation<Object> info = TypeExtractor.getForObject(key);
  			return info.createSerializer(executionConfig);
  		}
  		catch (Throwable t) {
  			throw new RuntimeException("Could not create key serializer for type " + key);
  		}
  	}
docs: --------------------------------------------------------------------------------------------