diff --git a/src/probabilistic/CardinalityCounter.cc b/src/probabilistic/CardinalityCounter.cc
index 84c5c91de1..4014b3de57 100644
--- a/src/probabilistic/CardinalityCounter.cc
+++ b/src/probabilistic/CardinalityCounter.cc
@@ -134,6 +134,10 @@ void CardinalityCounter::AddElement(uint64_t hash) {
  **/
 double CardinalityCounter::Size() const {
     double answer = 0;
+
+    if ( m == 0 )
+        return -1.0;
+
     for ( unsigned int i = 0; i < m; i++ )
         answer += pow(2, -((int)buckets[i]));
 
diff --git a/src/probabilistic/CardinalityCounter.h b/src/probabilistic/CardinalityCounter.h
index 68ac83cd46..5663530c93 100644
--- a/src/probabilistic/CardinalityCounter.h
+++ b/src/probabilistic/CardinalityCounter.h
@@ -75,7 +75,8 @@ public:
      * Get the current estimated number of elements in the data
      * structure
      *
-     * @return Estimated number of elements
+     * @return Estimated number of elements. Returns -1.0 if there are
+     * zero buckets.
      **/
     double Size() const;