diff --git a/src/probabilistic/CardinalityCounter.cc b/src/probabilistic/CardinalityCounter.cc
index 3e274da886..ed9f4ae078 100644
--- a/src/probabilistic/CardinalityCounter.cc
+++ b/src/probabilistic/CardinalityCounter.cc
@@ -10,7 +10,7 @@
 
 using namespace probabilistic;
 
-int CardinalityCounter::OptimalB(double error, double confidence)
+int CardinalityCounter::OptimalB(double error, double confidence) const
 	{
 	double initial_estimate = 2 * (log(1.04) - log(error)) / log(2);
 	int answer = (int) floor(initial_estimate);
@@ -87,7 +87,7 @@ CardinalityCounter::~CardinalityCounter()
 	delete [] buckets;
 	}
 
-uint8_t CardinalityCounter::Rank(uint64 hash_modified)
+uint8_t CardinalityCounter::Rank(uint64 hash_modified) const
 	{
 	uint8_t answer = 0;
 
diff --git a/src/probabilistic/CardinalityCounter.h b/src/probabilistic/CardinalityCounter.h
index a2aeea50eb..1d1e581d5d 100644
--- a/src/probabilistic/CardinalityCounter.h
+++ b/src/probabilistic/CardinalityCounter.h
@@ -147,7 +147,7 @@ private:
 	 *
 	 * @return minimal B-value satisfying the error-rate under confidence. 
 	 */
-	int OptimalB(double error, double confidence);
+	int OptimalB(double error, double confidence) const;
 
 	/**
 	 * Determines at which index (counted from the back) the first one-bit
@@ -158,7 +158,7 @@ private:
 	 *
 	 * @returns index of first one-bit
 	 */
-	uint8_t Rank(uint64 hash_modified);
+	uint8_t Rank(uint64 hash_modified) const;
 
 	/**
 	 * This is the number of buckets that will be stored. The standard