diff --git a/samoa-api/src/main/java/org/apache/samoa/evaluation/F1ClassificationPerformanceEvaluator.java b/samoa-api/src/main/java/org/apache/samoa/evaluation/F1ClassificationPerformanceEvaluator.java
index b59480a..3046360 100644
--- a/samoa-api/src/main/java/org/apache/samoa/evaluation/F1ClassificationPerformanceEvaluator.java
+++ b/samoa-api/src/main/java/org/apache/samoa/evaluation/F1ClassificationPerformanceEvaluator.java
@@ -33,7 +33,7 @@
         ClassificationPerformanceEvaluator {
 
     private static final long serialVersionUID = 1L;
-    protected int numClasses;
+    protected int numClasses = -1;
 
     protected long[] support;
     protected long[] truePos;
@@ -63,6 +63,7 @@
 
     @Override
     public void addResult(Instance inst, double[] classVotes) {
+        if (numClasses==-1) reset(inst.numClasses());
         int trueClass = (int) inst.classValue();
         this.support[trueClass] += 1;
         int predictedClass = Utils.maxIndex(classVotes);