diff --git a/grc/foo_packet_pad.xml b/grc/foo_packet_pad.xml
index 382b796..8b71562 100644
--- a/grc/foo_packet_pad.xml
+++ b/grc/foo_packet_pad.xml
@@ -37,6 +37,9 @@
 		<type>int</type>
 	</param>
 
+        <check>$pad_front >= 0</check>
+        <check>$pad_tail >= 0</check>
+
 	<sink>
 		<name>in</name>
 		<type>complex</type>
diff --git a/include/foo/packet_pad.h b/include/foo/packet_pad.h
index 5d751c7..81970da 100644
--- a/include/foo/packet_pad.h
+++ b/include/foo/packet_pad.h
@@ -28,7 +28,7 @@ class FOO_API packet_pad : virtual public gr::block
 public:
 
 	typedef boost::shared_ptr<packet_pad> sptr;
-	static sptr make(bool debug = false, int pad_front = 0, int pad_tail = 0);
+	static sptr make(bool debug = false, unsigned int pad_front = 0, unsigned int pad_tail = 0);
 
 };
 
diff --git a/lib/packet_pad_impl.cc b/lib/packet_pad_impl.cc
index 50e3f58..8fc2760 100644
--- a/lib/packet_pad_impl.cc
+++ b/lib/packet_pad_impl.cc
@@ -29,16 +29,13 @@ static const int SYMBOL_LENGTH = 80;
 #define dout d_debug && std::cout
 
 public:
-packet_pad_impl(bool debug, int pad_front, int pad_tail) : block("packet_pad",
+packet_pad_impl(bool debug, unsigned int pad_front, unsigned int pad_tail) : block("packet_pad",
 			gr::io_signature::make(1, 1, sizeof(gr_complex)),
 			gr::io_signature::make(1, 1, sizeof(gr_complex))),
 			d_debug(debug), d_pad_front(pad_front),
 			d_pad_tail(pad_tail), d_front_left(0),
 			d_tail_left(0), d_frame_left(0), d_state(IDLE) {
 
-	assert(pad_front >= 0);
-	assert(pad_tail >= 0);
-
 	set_relative_rate(1);
 	set_output_multiple(SYMBOL_LENGTH);
 	set_tag_propagation_policy(block::TPP_DONT);
@@ -232,6 +229,6 @@ private:
 };
 
 packet_pad::sptr
-packet_pad::make(bool debug, int pad_front, int pad_tail) {
+packet_pad::make(bool debug, unsigned int pad_front, unsigned int pad_tail) {
 	return gnuradio::get_initial_sptr(new packet_pad_impl(debug, pad_front, pad_tail));
 }