diff --git a/src/shogun/lib/tapkee/neighbors/covertree_point.hpp b/src/shogun/lib/tapkee/neighbors/covertree_point.hpp
index e42ab3e0916..bedb5350bb8 100644
--- a/src/shogun/lib/tapkee/neighbors/covertree_point.hpp
+++ b/src/shogun/lib/tapkee/neighbors/covertree_point.hpp
@@ -124,8 +124,8 @@ struct CoverTreePoint
 	ScalarType norm_;
 }; /* struct JLCoverTreePoint */
 
-template <bool, class RandomAccessIterator, class Callback> 
-struct distance_impl_if_kernel;
+template <class Type, class RandomAccessIterator, class Callback> 
+struct distance_impl;
 
 /** Functions declared out of the class definition to respect CoverTree 
  *  structure */
@@ -138,11 +138,13 @@ inline ScalarType distance(Callback& cb, const CoverTreePoint<RandomAccessIterat
 	if (l.iter_==r.iter_)
 		return 0.0;
 
-	return distance_impl_if_kernel<Callback::is_kernel,RandomAccessIterator,Callback>()(cb,l,r,upper_bound);
+	return distance_impl<typename Callback::type,RandomAccessIterator,Callback>()(cb,l,r,upper_bound);
 }
 
+struct KernelType;
+
 template <class RandomAccessIterator, class Callback>
-struct distance_impl_if_kernel<true,RandomAccessIterator,Callback>
+struct distance_impl<KernelType,RandomAccessIterator,Callback>
 {
 	inline ScalarType operator()(Callback& cb, const CoverTreePoint<RandomAccessIterator>& l,
                                  const CoverTreePoint<RandomAccessIterator>& r, ScalarType /*upper_bound*/)
@@ -151,8 +153,10 @@ struct distance_impl_if_kernel<true,RandomAccessIterator,Callback>
 	}
 };
 
+struct DistanceType;
+
 template <class RandomAccessIterator, class Callback>
-struct distance_impl_if_kernel<false,RandomAccessIterator,Callback>
+struct distance_impl<DistanceType,RandomAccessIterator,Callback>
 {
 	inline ScalarType operator()(Callback& cb, const CoverTreePoint<RandomAccessIterator>& l,
                                  const CoverTreePoint<RandomAccessIterator>& r, ScalarType /*upper_bound*/)
diff --git a/src/shogun/lib/tapkee/neighbors/neighbors.hpp b/src/shogun/lib/tapkee/neighbors/neighbors.hpp
index 67d9164d430..b783d76264f 100644
--- a/src/shogun/lib/tapkee/neighbors/neighbors.hpp
+++ b/src/shogun/lib/tapkee/neighbors/neighbors.hpp
@@ -36,6 +36,10 @@ struct distances_comparator
 	}
 };
 
+struct KernelType 
+{
+};
+
 template <class RandomAccessIterator, class Callback>
 struct KernelDistance
 {
@@ -48,11 +52,13 @@ struct KernelDistance
 	{
 		return sqrt(callback.kernel(*l,*l) - 2*callback.kernel(*l,*r) + callback.kernel(*r,*r));
 	}
-	static const bool is_kernel;
+	typedef KernelType type;
 	Callback callback;
 };
-template <class RandomAccessIterator, class Callback>
-const bool KernelDistance<RandomAccessIterator, Callback>::is_kernel = true;
+
+struct DistanceType
+{
+};
 
 template <class RandomAccessIterator, class Callback>
 struct PlainDistance
@@ -66,11 +72,9 @@ struct PlainDistance
 	{
 		return callback.distance(*l,*r);
 	}
-	static const bool is_kernel;
+	typedef DistanceType type;
 	Callback callback;
 };
-template <class RandomAccessIterator, class Callback>
-const bool PlainDistance<RandomAccessIterator, Callback>::is_kernel = false;
 
 #ifdef TAPKEE_USE_LGPL_COVERTREE
 template <class RandomAccessIterator, class Callback>
diff --git a/src/shogun/lib/tapkee/neighbors/vptree.hpp b/src/shogun/lib/tapkee/neighbors/vptree.hpp
index e501491f280..f39c3ca3108 100644
--- a/src/shogun/lib/tapkee/neighbors/vptree.hpp
+++ b/src/shogun/lib/tapkee/neighbors/vptree.hpp
@@ -1,12 +1,12 @@
 /* This software is distributed under BSD 3-clause license (see LICENSE file).
  *
- * Copyright (c) 2012-2013 Sergey Lisitsyn
+ * Copyright (c) 2012-2013 Laurens van der Maaten, Sergey Lisitsyn
  */
 
 #ifndef TAPKEE_VPTREE_H_
 #define TAPKEE_VPTREE_H_
 
-/* Tapkee include */
+/* Tapkee includes */
 #include <shogun/lib/tapkee/tapkee_defines.hpp>
 /* End of Tapkee includes */
 
@@ -20,8 +20,8 @@ namespace tapkee
 namespace tapkee_internal
 {
 
-template<bool, class RandomAccessIterator, class DistanceCallback> 
-struct compare_if_kernel;
+template<class Type, class RandomAccessIterator, class DistanceCallback> 
+struct compare_impl;
 
 template<class RandomAccessIterator, class DistanceCallback>
 struct DistanceComparator
@@ -32,13 +32,15 @@ struct DistanceComparator
 		callback(c), item(i) {}
 	inline bool operator()(const RandomAccessIterator& a, const RandomAccessIterator& b)
 	{
-		return compare_if_kernel<DistanceCallback::is_kernel,RandomAccessIterator,DistanceCallback>()
+		return compare_impl<typename DistanceCallback::type,RandomAccessIterator,DistanceCallback>()
 			(callback,item,a,b);
 	}
 };
 
+struct KernelType;
+
 template<class RandomAccessIterator, class DistanceCallback> 
-struct compare_if_kernel<true,RandomAccessIterator,DistanceCallback>
+struct compare_impl<KernelType,RandomAccessIterator,DistanceCallback>
 {
 	inline bool operator()(DistanceCallback& callback, const RandomAccessIterator& item,
 	                       const RandomAccessIterator& a, const RandomAccessIterator& b)
@@ -47,8 +49,10 @@ struct compare_if_kernel<true,RandomAccessIterator,DistanceCallback>
 	}
 };
 
+struct DistanceType;
+
 template<class RandomAccessIterator, class DistanceCallback> 
-struct compare_if_kernel<false,RandomAccessIterator,DistanceCallback>
+struct compare_impl<DistanceType,RandomAccessIterator,DistanceCallback>
 {
 	inline bool operator()(DistanceCallback& callback, const RandomAccessIterator& item,
 	                       const RandomAccessIterator& a, const RandomAccessIterator& b)
diff --git a/src/shogun/lib/tapkee/parameters/parameters.hpp b/src/shogun/lib/tapkee/parameters/parameters.hpp
index 7566f5633cf..b1bbd3a7755 100644
--- a/src/shogun/lib/tapkee/parameters/parameters.hpp
+++ b/src/shogun/lib/tapkee/parameters/parameters.hpp
@@ -14,10 +14,6 @@
 #include <vector>
 #include <map>
 
-using std::vector;
-using std::string;
-using std::stringstream;
-
 namespace tapkee
 {
 
@@ -37,7 +33,7 @@ struct Message
 		return ss.str();
 	}
 
-	stringstream ss;
+	std::stringstream ss;
 };
 
 class ParametersSet;
@@ -53,6 +49,7 @@ class Parameter
 
 	template <typename T>
 	Parameter(const ParameterName& pname, const T& value) : 
+		valid(true), invalidity_reason(),
 		parameter_name(pname), keeper(tapkee_internal::ValueKeeper(value))
 	{
 	}
@@ -65,11 +62,15 @@ class Parameter
 		return Parameter(name, value);
 	}
 
-	Parameter() : parameter_name("unknown"), keeper(tapkee_internal::ValueKeeper())
+	Parameter() : 
+		valid(false), invalidity_reason(),
+		parameter_name("unknown"), keeper(tapkee_internal::ValueKeeper())
 	{
 	}
 
-	Parameter(const Parameter& p) : parameter_name(p.name()), keeper(p.keeper)
+	Parameter(const Parameter& p) : 
+		valid(p.valid), invalidity_reason(p.invalidity_reason),
+		parameter_name(p.name()), keeper(p.keeper)
 	{
 	}
 
@@ -90,6 +91,10 @@ class Parameter
 	template <typename T>
 	inline operator T()
 	{
+		if (!valid)
+		{
+			throw wrong_parameter_error(invalidity_reason);
+		}
 		try 
 		{
 			return getValue<T>();
@@ -192,8 +197,17 @@ class Parameter
 		return keeper.isTypeCorrect<T>();
 	}
 
+	inline void invalidate(const std::string& reason)
+	{
+		valid = false;
+		invalidity_reason = reason;
+	}
+
 private:
 
+	bool valid;
+	std::string invalidity_reason;
+
 	ParameterName parameter_name;
 
 	tapkee_internal::ValueKeeper keeper; 
@@ -205,16 +219,10 @@ class CheckedParameter
 
 public:
 
-	explicit CheckedParameter(const Parameter& p) : parameter(p)
+	explicit CheckedParameter(Parameter& p) : parameter(p)
 	{
 	}
 
-	template <typename T>
-	inline operator T() const
-	{
-		return parameter.getValue<T>();
-	}
-	
 	inline operator const Parameter&()
 	{
 		return parameter;
@@ -237,11 +245,11 @@ class CheckedParameter
 	{
 		if (!parameter.isInRange(lower, upper))
 		{
-			std::string error_message = 
-				(Message() << "Value " << parameter.name() << " " 
+			std::string reason = 
+				(Message() << "Value of " << parameter.name() << " " 
 				 << parameter.getValue<T>() << " doesn't fit the range [" << 
 				    lower << ", " << upper << ")");
-			throw tapkee::wrong_parameter_error(error_message);
+			parameter.invalidate(reason);
 		}
 		return *this;
 	}
@@ -250,9 +258,9 @@ class CheckedParameter
 	{
 		if (!parameter.isPositive())
 		{
-			std::string error_message = 
+			std::string reason = 
 				(Message() << "Value of " << parameter.name() << " is not positive");
-			throw tapkee::wrong_parameter_error(error_message);
+			parameter.invalidate(reason);
 		}
 		return *this;
 	}
@@ -261,9 +269,9 @@ class CheckedParameter
 	{
 		if (!parameter.isNonNegative())
 		{
-			std::string error_message = 
+			std::string reason = 
 				(Message() << "Value of " << parameter.name() << " is negative");
-			throw tapkee::wrong_parameter_error(error_message);
+			parameter.invalidate(reason);
 		}
 		return *this;
 	}
@@ -271,7 +279,7 @@ class CheckedParameter
 
 private:
 
-	Parameter parameter;
+	Parameter& parameter;
 
 };