[TYPO3-50-general] Extend Image content type error

[Piseth Sok]

Hi All,

I want to create custom content type that can upload image.
I have extended Image content type but problem with save.
I got this message:

Exception while property mapping for target type 
"TYPO3\TYPO3CR\Domain\Model\NodeInterface", at property path "": 
Exception while property mapping for target type 
"TYPO3\Media\Domain\Model\ImageVariant", at property path "": No 
converter found which can be used to convert from "string" to 
"TYPO3\Media\Domain\Model\ImageVariant".

Anyone has solution about this?

Thanks,
Piseth