diff --git a/owlps-positioning/TODO b/owlps-positioning/TODO
index e8c44fb..d148df6 100644
--- a/owlps-positioning/TODO
+++ b/owlps-positioning/TODO
@@ -31,10 +31,6 @@
    les Waypoint. Si oui, faut-il aussi les enlever des listes dans
    Stock ? (Pour l'instant ils ne sont pas dans Stock.)
 
-- AccessPoint
-   ° Lancer une exception si le canal Wi-Fi est mauvais (ou
-   directement dans PosUtil::channel_to_frequency() ?).
-
 - ReferencePoint
    ° La liste des requêtes devrait être un unordered_set (et pas un
    vector), pour garantir l'unicité des entrées.
diff --git a/owlps-positioning/src/posexcept.cc b/owlps-positioning/src/posexcept.cc
index 717187a..1f1fcde 100644
--- a/owlps-positioning/src/posexcept.cc
+++ b/owlps-positioning/src/posexcept.cc
@@ -51,6 +51,15 @@ bad_direction::bad_direction(const char direction) throw()
 }
 
 
+bad_channel::bad_channel(const unsigned long channel) throw()
+{
+  ostringstream message ;
+  message << "`" << channel
+          << "` is not a valid channel value!" ;
+  explanation = message.str() ;
+}
+
+
 null_input_medium::null_input_medium() throw():
   posexcept("The input medium is not initialised!") {}
 
diff --git a/owlps-positioning/src/posexcept.hh b/owlps-positioning/src/posexcept.hh
index 27ba7cc..80a78fc 100644
--- a/owlps-positioning/src/posexcept.hh
+++ b/owlps-positioning/src/posexcept.hh
@@ -79,6 +79,13 @@ public:
 } ;
 
 
+class bad_channel: public posexcept
+{
+public:
+  bad_channel(const unsigned long channel) throw() ;
+} ;
+
+
 class null_input_medium: public posexcept
 {
 public:
diff --git a/owlps-positioning/src/posutil.cc b/owlps-positioning/src/posutil.cc
index 060d4ef..1b2b3b0 100644
--- a/owlps-positioning/src/posutil.cc
+++ b/owlps-positioning/src/posutil.cc
@@ -1,8 +1,9 @@
 #include "posutil.hh"
+#include "posexcept.hh"
 
 
 
-/* Wi-Fi channel frequencies in MHz */
+/* Wi-Fi channel frequencies in Hz */
 #define CHANNEL_1  2412000000ul
 #define CHANNEL_2  2417000000ul
 #define CHANNEL_3  2422000000ul
@@ -21,12 +22,12 @@
 
 
 /**
- * @param channel A IEEE 802.11 channel.
- * @return The frequency in Hz or 0 if the channel is wrong.
- * Note that if \em channel is a frequency in Hz, this function will
- * consider it as a wrong value and return 0.
+ * @param channel A IEEE 802.11 channel or frequency in Hz.
+ * @return The frequency in Hz.
+ * @throw malformed_input_data if \em channel is not a valid channel or
+ * frequency value.
  */
-unsigned long PosUtil::wifi_channel_to_hz(const unsigned int &channel)
+unsigned long PosUtil::wifi_channel_to_hz(const unsigned long &channel)
 {
   switch (channel)
     {
@@ -74,5 +75,6 @@ unsigned long PosUtil::wifi_channel_to_hz(const unsigned int &channel)
       return CHANNEL_14 ;
     }
 
-  return 0 ; // Error: wrong channel value
+  // Error: wrong channel value
+  throw bad_channel(channel) ;
 }
diff --git a/owlps-positioning/src/posutil.hh b/owlps-positioning/src/posutil.hh
index aa66643..fd1e274 100644
--- a/owlps-positioning/src/posutil.hh
+++ b/owlps-positioning/src/posutil.hh
@@ -11,7 +11,7 @@ public:
   /** @name Wi-Fi */
   //@{
   /// Converts a Wi-Fi channel to the corresponding frequency in Hz
-  static unsigned long wifi_channel_to_hz(const unsigned int &channel) ;
+  static unsigned long wifi_channel_to_hz(const unsigned long &channel) ;
   //@}
 } ;