References deposited without "keys" appear as empty brackets in API results

[ssmulyan]

A metadata user reported finding "empty reference objects" for a number of DOIs. Based on the example he provided, it looks like those correspond to references that were deposited without keys.

For example, metadata for 10.1186/2008-2231-20-88 was last updated with submission 1355381104 which included these among its references.

<citation key="10.1186/2008-2231-20-88-B20">
<doi>10.1186/1472-698X-5-5</doi>
</citation>
<citation key="-">
<doi>10.1108/09526860510612207</doi>
</citation>
<citation key="-">
<doi>10.3923/ijp.2012.586.589</doi>
</citation>
<citation key="-">
<doi>10.1016/0749-5978(91)90020-T</doi>
</citation>
<citation key="-">
<doi>10.1348/135910705X66043</doi>
</citation>
<citation key="10.1186/2008-2231-20-88-B31">
<doi>10.1111/j.1365-2753.2011.01690.x</doi>
</citation>

The results in the REST API here show all the references with citation key="-" as empty brackets
http://api.crossref.org/works/10.1186/2008-2231-20-88

{
"key": "10.1186/2008-2231-20-88-B20",
"DOI": "10.1186/1472-698X-5-5",
"doi-asserted-by": "publisher"
},
{},
{},
{},
{},
{
"key": "10.1186/2008-2231-20-88-B31",
"DOI": "10.1111/j.1365-2753.2011.01690.x",
"doi-asserted-by": "publisher"

screenshots of the above also attached.