diff --git a/owlps-positioning/src/accesspointsreadercsv.cc b/owlps-positioning/src/accesspointsreadercsv.cc
index b6c7508..a33aa52 100644
--- a/owlps-positioning/src/accesspointsreadercsv.cc
+++ b/owlps-positioning/src/accesspointsreadercsv.cc
@@ -38,7 +38,7 @@ void AccessPointsReaderCSV::process_access_point_line()
   if (! file.read_point3d(coord))
     throw malformed_input_data("Cannot read access point coordinates!") ;
 
-  long frequency ;
+  unsigned long frequency ;
   if (! file.read_field(frequency))
     throw malformed_input_data("Cannot read access point frequency!") ;