diff --git a/src/Instrumentation/newnavradio.cxx b/src/Instrumentation/newnavradio.cxx
index c6db1ce88..6c65d2e02 100644
--- a/src/Instrumentation/newnavradio.cxx
+++ b/src/Instrumentation/newnavradio.cxx
@@ -783,7 +783,7 @@ void GS::update( double dt, const SGGeod & aircraftPosition )
   double offset = _targetGlideslope_deg - gsDirect;
   if( offset < 0.0 )
     offset = _targetGlideslope_deg/2 * sawtooth(2.0*offset/_targetGlideslope_deg);
-  assert( false == isnan(offset) );
+    assert( false == SGMisc<double>::isNaN(offset) );
 // GS is documented to be 1.4 degrees thick, 
 // i.e. plus or minus 0.7 degrees from the midline:
   _glideslopeOffset_norm = SGMiscd::clip(offset/0.7, -1.0, 1.0);