diff --git a/src/p_acs.cpp b/src/p_acs.cpp
index 8598c9aeb..07b1662be 100644
--- a/src/p_acs.cpp
+++ b/src/p_acs.cpp
@@ -936,6 +936,8 @@ void FBehavior::SerializeVarSet (FArchive &arc, SDWORD *vars, int max)
 	{
 		SDWORD truelast;
 
+		memset (vars, 0, max*sizeof(*vars));
+
 		arc << last;
 		if (last == 0)
 		{
@@ -950,8 +952,6 @@ void FBehavior::SerializeVarSet (FArchive &arc, SDWORD *vars, int max)
 			last = max;
 		}
 
-		memset (vars, 0, max*sizeof(*vars));
-
 		while (first < last)
 		{
 			arc << vars[first];