Why see an 'memory not enough' errors when full text search ? (multiterm - hitssearch - full-text search)

- using dll : Sdl.Multiterm.TMP.Interop
- target .sdltb entries count = 3900 rows over
- uniqueness : no problem when the number of entries is small.
- test system : cpu 2 core, mem 8gb
- using kb : sdl trados sp2, sdl multitum 2014 sp2 - 11.2.1379.0

---.Net(C#) Code--------------------------------------------------------------
using smti = Sdl.MultiTerm.TMO.Interop;

smti.MultiSearch tbSearch = new smti.MultiSearch();
tbSearch.Direction = smti.MtSearchDirection.mtSearchDown;
tbSearch.MaximumHits = maxHits;
tbSearch.SearchExpression = expression;
tbSearch.SearchType = smti.MtSearchType.mtHierarchial;
tbSearch.AddSearchTermbase( this.Termbase, srcDspLanguage, srcDspLanguage );
tbSearch.FullTextSearch = true;

smti.HitTerms hits = tbSearch.Execute(); << return result count '0'

foreach ( smti.SearchInfo si in tbSearch.SearchInfo )
{
int errCode = si.ErrorCode; << return "-2147217900"
string errMsg = si.ErrorMessage; << return "memory not enough'"
}

Help me...