diff --git a/jsprit-core/src/main/java/com/graphhopper/jsprit/core/util/UnassignedJobReasonTracker.java b/jsprit-core/src/main/java/com/graphhopper/jsprit/core/util/UnassignedJobReasonTracker.java
index a4577a05..0ab34dec 100644
--- a/jsprit-core/src/main/java/com/graphhopper/jsprit/core/util/UnassignedJobReasonTracker.java
+++ b/jsprit-core/src/main/java/com/graphhopper/jsprit/core/util/UnassignedJobReasonTracker.java
@@ -29,8 +29,11 @@ import java.util.*;
  */
 public class UnassignedJobReasonTracker implements JobUnassignedListener {
 
+    private final static String NO_REASON = "cannot determine a particular reason";
+
     public static String getMostLikelyFailedConstraintName(Frequency failedConstraintNamesFrequency) {
-        if (failedConstraintNamesFrequency == null) return "no reason found";
+        if (failedConstraintNamesFrequency == null || failedConstraintNamesFrequency.getUniqueCount() == 0)
+            return NO_REASON;
         Iterator<Map.Entry<Comparable<?>, Long>> entryIterator = failedConstraintNamesFrequency.entrySetIterator();
         long maxCount = 0;
         String mostLikely = null;
@@ -54,6 +57,7 @@ public class UnassignedJobReasonTracker implements JobUnassignedListener {
     Set<String> failedConstraintNamesToBeIgnored = new HashSet<>();
 
     public UnassignedJobReasonTracker() {
+        codesToHumanReadableReason.put(-1, NO_REASON);
         codesToHumanReadableReason.put(1, "cannot serve required skill");
         codesToHumanReadableReason.put(2, "cannot be visited within time window");
         codesToHumanReadableReason.put(3, "does not fit into any vehicle due to capacity");
@@ -165,8 +169,9 @@ public class UnassignedJobReasonTracker implements JobUnassignedListener {
      * @return
      */
     public String getMostLikelyReason(String jobId) {
-        if (!this.failedConstraintNamesFrequencyMapping.containsKey(jobId)) return "no reason found";
+        if (!this.failedConstraintNamesFrequencyMapping.containsKey(jobId)) return codesToHumanReadableReason.get(-1);
         Frequency reasons = this.failedConstraintNamesFrequencyMapping.get(jobId);
+
         String mostLikelyReason = getMostLikelyFailedConstraintName(reasons);
         int code = toCode(mostLikelyReason);
         if (code == -1) return mostLikelyReason;