diff --git a/core/smn_halflife.cpp b/core/smn_halflife.cpp
index 31a0aa6d..40f16678 100644
--- a/core/smn_halflife.cpp
+++ b/core/smn_halflife.cpp
@@ -459,6 +459,8 @@ static cell_t GuessSDKVersion(IPluginContext *pContext, const cell_t *params)
 		return 15;
 	case SOURCE_ENGINE_ORANGEBOX:
 		return 30;
+	case SOURCE_ENGINE_ORANGEBOXVALVE:
+		return 35;
 	case SOURCE_ENGINE_LEFT4DEAD:
 		return 40;
 	case SOURCE_ENGINE_LEFT4DEAD2: