Adcenter Error Code 1514 : The Negative keywords requires partial match bid

Got this error when dealing with a keyword update through the Adcenter API. Its a shame that Microsoft has no documentation on this error and none the less anywhere on the internet, hence the drive to blog this. Here's a brief background on Adcenter Negative Keywords:

http://msdn.microsoft.com/en-us/library/bb545038.aspx

So what the error basically means is that "Negative keywords are only allowed on keywords that have a broad or phrase match bid specified". If you try to add/update a keyword with "Not Keywords" and only exact bid specified then you would get this error. In my case this was being caused by a different scenario. We use Apache Axis for our webservice calls and Axis treats a blank string as a valid content string when compared to a null string. So even if the "Not Keywords" was set to blank it was generating the "Negative Keyword" soap element. See below for SOAP source. In the soap below the broad and phrase bid is specified as 0's and the "NegativeKeywords" element is constructed making MSN think that there are negative keywords associated with this keyword and hence the error. I resolved this error by setting the string to NULL instead of a blank ("''").

<?xml version="1.0" encoding="UTF-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<soapenv:Header><ns1:ApplicationToken soapenv:actor="http://schemas.xmlsoap.org/soap/actor/next" soapenv:mustUnderstand="0" xmlns:ns1="https://adcenter.microsoft.com/api/advertiser/v5"/><ns2:DeveloperToken soapenv:actor="http://schemas.xmlsoap.org/soap/actor/next" soapenv:mustUnderstand="0" xmlns:ns2="https://adcenter.microsoft.com/api/advertiser/v5"><ns2:Value>xxxx</ns2:Value></ns2:DeveloperToken><ns3:UserCredentials soapenv:actor="http://schemas.xmlsoap.org/soap/actor/next" soapenv:mustUnderstand="0" xmlns:ns3="https://adcenter.microsoft.com/api/advertiser/v5"><ns3:Password>xxxxxx</ns3:Password><ns3:Username>xxxxx</ns3:Username></ns3:UserCredentials></soapenv:Header>
<soapenv:Body>
<UpdateKeywordsRequest xmlns="https://adcenter.microsoft.com/api/advertiser/v5"><AdGroupId>xxxxx</AdGroupId><Keywords>
<Keyword><BroadMatchBid>0.0</BroadMatchBid><ExactMatchBid>0.5</ExactMatchBid><Id>xxxxx</Id><NegativeKeywords><ns4:string xmlns:ns4="http://schemas.microsoft.com/2003/10/Serialization/Arrays"></ns4:string></NegativeKeywords><Param1>xxxx</Param1><Param2></Param2><Param3></Param3><PhraseMatchBid>0.0</PhraseMatchBid></Keyword>
<Keyword><BroadMatchBid>0.0</BroadMatchBid><ExactMatchBid>0.5</ExactMatchBid><Id>xxxxxx</Id><NegativeKeywords><ns5:string xmlns:ns5="http://schemas.microsoft.com/2003/10/Serialization/Arrays"></ns5:string></NegativeKeywords><Param1>xxxxxxx</Param1><Param2></Param2><Param3></Param3><PhraseMatchBid>0.0</PhraseMatchBid></Keyword>
</Keywords>
</UpdateKeywordsRequest>
</soapenv:Body>
</soapenv:Envelope> 

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Body><s:Fault><faultcode>s:Client</faultcode><faultstring xml:lang="en-US">Invalid client data. Check the SOAP fault details for more information</faultstring>
<detail>
<EditorialApiFaultDetail xmlns="https://adcenter.microsoft.com/api/advertiser/v5" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<BatchErrors>
<BatchError><Code>1514</Code><Details i:nil="true"/><Index>0</Index><Message>The Negative keywords requires partial match bid.</Message></BatchError>
<BatchError><Code>1514</Code><Details i:nil="true"/><Index>1</Index><Message>The Negative keywords requires partial match bid.</Message></BatchError>
</BatchErrors>
<OperationErrors/>
<TrackingId>xxxxxxx</TrackingId>
<EditorialErrors/></EditorialApiFaultDetail>
</detail>
</s:Fault></s:Body>
</s:Envelope>