diff --git a/indra/newview/llmeshrepository.cpp b/indra/newview/llmeshrepository.cpp
index 78ac65bcb5..b97bc678b2 100644
--- a/indra/newview/llmeshrepository.cpp
+++ b/indra/newview/llmeshrepository.cpp
@@ -3995,10 +3995,11 @@ void LLMeshRepository::uploadModel(std::vector<LLModelInstance>& data, LLVector3
 
 S32 LLMeshRepository::getMeshSize(const LLUUID& mesh_id, S32 lod)
 {
-	if (mThread)
+	if (mThread && mesh_id.notNull())
 	{
+		LLMutexLock lock(mThread->mHeaderMutex);
 		LLMeshRepoThread::mesh_header_map::iterator iter = mThread->mMeshHeader.find(mesh_id);
-		if (iter != mThread->mMeshHeader.end())
+		if (iter != mThread->mMeshHeader.end() && mThread->mMeshHeaderSize[mesh_id] > 0)
 		{
 			LLSD& header = iter->second;