Error switching to SOLR, with Exerpience Analytics/Reduce/Agent (IFieldMapEx error)

I recently encountered an error when trying to switch my Sitecore instance over to using SOLR. The error is pasted below, which turned up ZERO results googling. Always fun when that happens.

Turned out to be just a mismatch in versions. I'm running Sitecore 8.1 rev160302, which at the time I installed it was the most recent version. I wasn't paying attention when I grabbed the SOLR Support Package, and I just went to the most recent version of Sitecore... which is now Sitecore 8.1 rev 160519. Turns out versions really do matter!

Dumb error on my part, but I'd figure I'd make a post about it just in case anyone else makes the same mistake, so they aren't treated to an empty google search.

3760 13:10:59 ERROR Exception when executing agent experienceAnalytics/reduce/agent
Exception: System.Reflection.TargetInvocationException
Message: Exception has been thrown by the target of an invocation.
Source: mscorlib
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at Sitecore.Reflection.ReflectionUtil.CreateObject(Type type, Object[] parameters)
at Sitecore.Configuration.Factory.CreateFromTypeName(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.CreateObject(String configPath, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateFromReference(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.GetInnerObject(XmlNode paramNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.GetConstructorParameters(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateFromTypeName(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.CreateObject(String configPath, String[] parameters, Boolean assert)
at Sitecore.Analytics.Core.BackgroundService.CreateAgent()
at Sitecore.Analytics.Core.BackgroundService.ExecuteAgent()
at Sitecore.Analytics.Core.BackgroundService.Run()

Nested Exception

Exception: System.Reflection.TargetInvocationException
Message: Exception has been thrown by the target of an invocation.
Source: mscorlib
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at Sitecore.Reflection.ReflectionUtil.CreateObject(Type type, Object[] parameters)
at Sitecore.Configuration.Factory.CreateFromTypeName(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.CreateObject[T](XmlNode configNode)
at Sitecore.ExperienceAnalytics.Reduce.ReduceContainer.Repositories.GetDimensionDefinitionService()
at Sitecore.ExperienceAnalytics.Reduce.ReduceWorker..ctor(String connectionString)
at Sitecore.ExperienceAnalytics.Reduce.ReduceManager..ctor(String connectionStringName, String retentionDays, ILogger logger)

Nested Exception

Exception: System.Reflection.TargetInvocationException
Message: Exception has been thrown by the target of an invocation.
Source: mscorlib
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at Sitecore.Reflection.ReflectionUtil.CreateObject(Type type, Object[] parameters)
at Sitecore.ExperienceAnalytics.Core.Repositories.DimensionDefinitionService.CreateDimensionFromConfig(XmlElement childNode)
at Sitecore.ExperienceAnalytics.Core.Repositories.DimensionDefinitionService.LoadDimensionsFromConfig(String pathToConfigNode)

Nested Exception

Exception: System.Reflection.TargetInvocationException
Message: Exception has been thrown by the target of an invocation.
Source: mscorlib
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at Sitecore.Reflection.ReflectionUtil.CreateObject(Type type, Object[] parameters)
at Sitecore.Configuration.Factory.CreateFromTypeName(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.CreateObject(String configPath, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateFromReference(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.GetInnerObject(XmlNode paramNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.GetConstructorParameters(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateFromTypeName(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.CreateObject(String configPath, String[] parameters, Boolean assert)
at Sitecore.Marketing.Definitions.DefinitionManagerFactory.GetDefinitionManager[TDefinition](String targetRepository)
at Sitecore.ExperienceAnalytics.Aggregation.Dimensions.ByCampaignGroup..ctor(Guid dimensionId)

Nested Exception

Exception: System.Reflection.TargetInvocationException
Message: Exception has been thrown by the target of an invocation.
Source: mscorlib
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at Sitecore.Reflection.ReflectionUtil.CreateObject(Type type, Object[] parameters)
at Sitecore.Configuration.Factory.CreateFromTypeName(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.CreateObject(String configPath, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateFromReference(XmlNode configNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.GetInnerObject(XmlNode paramNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.AssignProperties(XmlNode configNode, String[] parameters, Object obj, Boolean assert, Boolean deferred, IFactoryHelper helper)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.GetInnerObject(XmlNode paramNode, String[] parameters, Boolean assert)
at Sitecore.Configuration.Factory.AssignProperties(XmlNode configNode, String[] parameters, Object obj, Boolean assert, Boolean deferred, IFactoryHelper helper)
at Sitecore.Configuration.Factory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
at Sitecore.Configuration.Factory.CreateObject(String configPath, String[] parameters, Boolean assert)
at Sitecore.ContentSearch.ContentSearchManager.get_SearchConfiguration()
at Sitecore.ContentSearch.ContentSearchManager.GetIndex(String name)
at Sitecore.Marketing.Search.CampaignDefinitionSearchProvider..ctor(String indexName)

Nested Exception

Exception: System.TypeLoadException
Message: Could not load type 'Sitecore.ContentSearch.IFieldMapEx' from assembly 'Sitecore.ContentSearch, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.
Source: Sitecore.ContentSearch.SolrProvider
at Sitecore.ContentSearch.SolrProvider.SolrIndexConfiguration.set_DocumentOptions(IDocumentBuilderOptions value)