diff --git a/algorithms/active/nlstar/src/main/java/de/learnlib/algorithms/nlstar/Row.java b/algorithms/active/nlstar/src/main/java/de/learnlib/algorithms/nlstar/Row.java
index c3ecd613cf..408864ba41 100644
--- a/algorithms/active/nlstar/src/main/java/de/learnlib/algorithms/nlstar/Row.java
+++ b/algorithms/active/nlstar/src/main/java/de/learnlib/algorithms/nlstar/Row.java
@@ -129,7 +129,9 @@ public List<Row<I>> getCoveredRows() {
     }
 
     boolean checkPrime() {
-        if (coveredRows.isEmpty()) {
+        if (contents.isEmpty()) {
+            prime = false;
+        } else if (coveredRows.isEmpty()) {
             prime = true;
         } else {
             BitSet aggContents = new BitSet();