diff --git a/src/CompHash.cc b/src/CompHash.cc
index cc4f440e06..2f1a41e311 100644
--- a/src/CompHash.cc
+++ b/src/CompHash.cc
@@ -407,7 +407,7 @@ int CompositeHash::SingleTypeKeySize(BroType* bt, const Val* v,
 
 	case TYPE_INTERNAL_STRING:
 		if ( ! v )
-			return 0;
+			return optional ? sz : 0;
 
 		// Factor in length field.
 		sz = SizeAlign(sz, sizeof(int));