diff --git a/iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/NodeBreakerVoltageLevel.java b/iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/NodeBreakerVoltageLevel.java
index 9de68df1721..73901a057d4 100644
--- a/iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/NodeBreakerVoltageLevel.java
+++ b/iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/NodeBreakerVoltageLevel.java
@@ -23,7 +23,6 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
-import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
@@ -400,11 +399,11 @@ BusExt getConnectableBus(int node) {
             });
             // if nothing found, just take the first bus
             if (connectableBus2[0] == null) {
-                Iterator<CalculatedBus> it = getBuses().iterator();
-                if (!it.hasNext()) {
-                    throw new AssertionError("Should not happen");
+                Collection<CalculatedBus> buses = getBuses();
+                if (buses.isEmpty()) { // if the whole voltage level is disconnected, return null
+                    return null;
                 }
-                return it.next();
+                return buses.iterator().next();
             }
             return connectableBus2[0];
         }
diff --git a/iidm/iidm-impl/src/test/java/com/powsybl/iidm/network/impl/NodeBreakerTest.java b/iidm/iidm-impl/src/test/java/com/powsybl/iidm/network/impl/NodeBreakerTest.java
index 9af742a0892..56cc485fb63 100644
--- a/iidm/iidm-impl/src/test/java/com/powsybl/iidm/network/impl/NodeBreakerTest.java
+++ b/iidm/iidm-impl/src/test/java/com/powsybl/iidm/network/impl/NodeBreakerTest.java
@@ -75,6 +75,9 @@ private static Network createIsolatedLoadNetwork() {
         VoltageLevel vl1 = s1.newVoltageLevel().setId("VL").setNominalV(1f)
                 .setTopologyKind(TopologyKind.NODE_BREAKER)
                 .add();
+        VoltageLevel vl2 = s1.newVoltageLevel().setId("VL2").setNominalV(1f)
+                .setTopologyKind(TopologyKind.NODE_BREAKER)
+                .add();
 
         vl1.getNodeBreakerView()
                 .setNodeCount(11)
@@ -150,6 +153,15 @@ private static Network createIsolatedLoadNetwork() {
                 .setNode2(2)
                 .setRetained(true)
                 .add();
+
+        vl2.getNodeBreakerView()
+                .setNodeCount(1);
+        vl2.newLoad()
+                .setId("L4")
+                .setNode(0)
+                .setP0(0)
+                .setQ0(0)
+                .add();
         return network;
     }
 
@@ -283,5 +295,9 @@ public void testIsolatedLoadBusBranch() {
         // load "L3" is not connected and has no connectable bus (the first bus is taken as connectable bus in this case)
         assertNull(getBus(network.getLoad("L3")));
         assertEquals("VL_0", getConnectableBus(network.getLoad("L3")).getId());
+
+        // load "L4" is not connected, has no connectable bus and is in a disconnected voltage level
+        assertNull(getBus(network.getLoad("L4")));
+        assertNull(getConnectableBus(network.getLoad("L4")));
     }
 }