diff --git a/jastadd-mquat-eris/src/main/java/de/tudresden/inf/st/mquat/eris/coupling/ERISAttribute.java b/jastadd-mquat-eris/src/main/java/de/tudresden/inf/st/mquat/eris/coupling/ERISAttribute.java
index 671f5feaf86df08f2e9b8047f8dc623a364cc57a..fbad9c6b03bd7de66e4213e2f6ec58e8032666f8 100644
--- a/jastadd-mquat-eris/src/main/java/de/tudresden/inf/st/mquat/eris/coupling/ERISAttribute.java
+++ b/jastadd-mquat-eris/src/main/java/de/tudresden/inf/st/mquat/eris/coupling/ERISAttribute.java
@@ -26,6 +26,7 @@ public class ERISAttribute {
     typeLength = jsonObject.getInt("typeLength");
     unique = jsonObject.getBoolean("unique");
     allowIndex = jsonObject.getBoolean("allowIndex");
+    allowIndex = allowIndex && notNull && notUndefined;
     this.container = container;
   }