Unknown field error when re-indexing Sitecore_marketing_asset_index_master using SOLR

Hi all, 

I haven't used SOLR much but I'm getting the below error when I re-index siteocre_marketing_asset_index_master (just this one. Everything else is fine). Has anyone experienced this issue before? 

I'm using Sitecore 8 Update 4. I'm using SOLR 5.3.0.0 (using Bitnami SOLR stack) and I have been following the instructions here: https://sitecore-community.github.io/docs/search/solr/Configuring-Solr-for-use-with-Sitecore-8/ (btw, thanks to all involved with that documentation. It was very helpful). 

Sitecore marketing asset master - throws error when re-indexed:
Job started: Index_Update_IndexName=sitecore_marketing_asset_index_master|#Exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> SolrNet.Exceptions.SolrConnectionException: <?xml version="1.0" encoding="UTF-8"?>
<response>
<lst name="responseHeader"><int name="status">400</int><int name="QTime">111</int></lst><lst name="error"><str name="msg">ERROR: [doc=sitecore://master/

{f8c69ccd-422c-40ac-843d-2e4144f16964}

?lang=uk-ua&ver=1&ndx=sitecore_marketing_asset_index_master] unknown field 'width_t_uk'</str><int name="code">400</int></lst>
</response>
---> System.Net.WebException: The remote server returned an error: (400) Bad Request.
at System.Net.HttpWebRequest.GetResponse()
at HttpWebAdapters.Adapters.HttpWebRequestAdapter.GetResponse()
at SolrNet.Impl.SolrConnection.GetResponse(IHttpWebRequest request)
at SolrNet.Impl.SolrConnection.PostStream(String relativeUrl, String contentType, Stream content, IEnumerable`1 parameters)
— End of inner exception stack trace —
at SolrNet.Impl.SolrConnection.PostStream(String relativeUrl, String contentType, Stream content, IEnumerable`1 parameters)
at SolrNet.Impl.SolrConnection.Post(String relativeUrl, String s)
at SolrNet.Impl.SolrBasicServer`1.SendAndParseHeader(ISolrCommand cmd)
at Sitecore.ContentSearch.SolrProvider.SolrBatchUpdateContext.AddRange(IEnumerable`1 group, Int32 groupSize)
at Sitecore.ContentSearch.SolrProvider.SolrBatchUpdateContext.Commit()
at Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.PerformRebuild(Boolean resetIndex, Boolean optimizeOnComplete, IndexingOptions indexingOptions, CancellationToken cancellationToken)
at Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.Rebuild(Boolean resetIndex, Boolean optimizeOnComplete)
— End of inner exception stack trace —
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Jobs.Job.ThreadEntry(Object state)

 

Any insights will be great!

Thanks,

Angel