diff --git a/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java b/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java
index c29a71e..d77cf26 100644
--- a/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java
+++ b/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java
@@ -86,9 +86,11 @@ public class DataStore {
 
 			final String value = tags.getValue(key);
 
-			final IntList docIds = valueToDocIds.computeIfAbsent(value, v -> new IntList());
-			synchronized (docIds) {
-				docIds.add(docId);
+			if (value != null) {
+				final IntList docIds = valueToDocIds.computeIfAbsent(value, v -> new IntList());
+				synchronized (docIds) {
+					docIds.add(docId);
+				}
 			}
 		}
 	}