ISO8601DateConverter could not parse date for empty element

<product>
<productId>10454</productId>
<title>Playlist 1</title>
<type>playlist</type>
<summary/>
<restrictedAccess>0</restrictedAccess>
<media>
<cover>XXXX</cover>
</media>
<artistList>
<artists></artists>
<offset>0</offset>
<count>1</count>
<total>1</total>
</artistList>
<label/>
<releaseDate/>
<flag>
<topvente>99</topvente>
<selection>99</selection>
<new>99</new>
</flag>
</product>

@Override
    public Object fromString(String str) {
        if (StringUtils.isNotBlank(str))
            return ((Calendar)super.fromString(str)).getTime();
        return null;
    }