diff --git a/samoa-instances/src/main/java/org/apache/samoa/instances/Attribute.java b/samoa-instances/src/main/java/org/apache/samoa/instances/Attribute.java
index 128ace7..4f49b43 100644
--- a/samoa-instances/src/main/java/org/apache/samoa/instances/Attribute.java
+++ b/samoa-instances/src/main/java/org/apache/samoa/instances/Attribute.java
@@ -243,8 +243,14 @@
       }
     }
     Integer val = (Integer) this.valuesStringAttribute.get(value);
+    
+    // in case this value was not on a list of unique values of nominal attribute yet, add it
+    // this means, the list of values can be extended with new entries arriving in the stream
     if (val == null) {
-      return -1;
+      int currentValueCount=this.valuesStringAttribute.size();
+      this.valuesStringAttribute.put(value,currentValueCount);
+      this.attributeValues.add(value);
+      return currentValueCount;
     } else {
       return val.intValue();
     }