i get these same two errors over and over in my event log.  Can anyone tell me what they mean, if they are a problem and how to fix them?

DNN 4.5.3

AssemblyVersion: -1
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL:
AbsoluteURL:
AbsoluteURLReferrer:
UserAgent:
DefaultDataProvider:
ExceptionGUID:
InnerException: Value cannot be null. Parameter name: type
FileName:
FileLineNumber: -1
FileColumnNumber: -1
Method:
StackTrace:
Message: System.ArgumentNullException: Value cannot be null. Parameter name: typeat System.Activator.CreateInstance(Type type, Boolean nonPublic)at DotNetNuke.Framework.Reflection.CreateObject(String TypeName, String CacheKey)at DotNetNuke.Services.Search.ModuleIndexer.GetModuleList(Int32 PortalID)
Source:
Server Name: NEWYORK

AssemblyVersion: -1
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL:
AbsoluteURL:
AbsoluteURLReferrer:
UserAgent:
DefaultDataProvider:
ExceptionGUID:
InnerException: Could not load type ''.
FileName:
FileLineNumber: -1
FileColumnNumber: -1
Method:
StackTrace:
Message: System.Web.HttpException: Could not load type ''. ---> System.ArgumentException: String cannot have zero length.at System.Reflection.Assembly.GetType(String name, Boolean throwOnError, Boolean ignoreCase)at System.Web.Compilation.BuildManager.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase)--- End of inner exception stack trace ---at System.Web.Compilation.BuildManager.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase)at DotNetNuke.Framework.Reflection.CreateType(String TypeName, String CacheKey, Boolean UseCache, Boolean IgnoreErrors)
Source:
Server Name: NEWYORK