diff --git a/libwvdrmengine/cdm/src/wv_content_decryption_module.cpp b/libwvdrmengine/cdm/src/wv_content_decryption_module.cpp
index 1e0fcce8..f7098e98 100644
--- a/libwvdrmengine/cdm/src/wv_content_decryption_module.cpp
+++ b/libwvdrmengine/cdm/src/wv_content_decryption_module.cpp
@@ -72,6 +72,7 @@ CdmResponseType WvContentDecryptionModule::CloseSession(
   CdmEngine* cdm_engine = GetCdmForSessionId(session_id);
   // TODO(rfrias): Avoid reusing the error codes from CdmEngine.
   if (!cdm_engine) return SESSION_NOT_FOUND_1;
+  AutoLock auto_lock(cdms_lock_);
   CdmResponseType sts = cdm_engine->CloseSession(session_id);
   cdm_engine->GetMetrics()->cdm_engine_close_session_.Increment(sts);
   if (sts == NO_ERROR) {