diff --git a/src/blackmisc/aviation/comsystem.cpp b/src/blackmisc/aviation/comsystem.cpp
index 64cbec110..f6c17eb72 100644
--- a/src/blackmisc/aviation/comsystem.cpp
+++ b/src/blackmisc/aviation/comsystem.cpp
@@ -137,7 +137,7 @@ namespace BlackMisc
 
         bool CComSystem::isWithinChannelSpacing(const CFrequency &setFrequency, const CFrequency &compareFrequency, CComSystem::ChannelSpacing channelSpacing)
         {
-            if (setFrequency.isNull()) { return false; }
+            if (setFrequency.isNull() || compareFrequency.isNull()) { return false; }
             if (setFrequency == compareFrequency) return true; // shortcut for many of such comparisons
             double channelSpacingKHz = 0.5 * CComSystem::channelSpacingToFrequencyKHz(channelSpacing);
             double compareFrequencyKHz = compareFrequency.value(CFrequencyUnit::kHz());
diff --git a/src/blackmisc/aviation/comsystem.h b/src/blackmisc/aviation/comsystem.h
index 6184c94c4..483eeb713 100644
--- a/src/blackmisc/aviation/comsystem.h
+++ b/src/blackmisc/aviation/comsystem.h
@@ -52,9 +52,9 @@ namespace BlackMisc
             //! Channel spacing frequency
             enum ChannelSpacing
             {
-                ChannelSpacing50KHz,
-                ChannelSpacing25KHz,
-                ChannelSpacing8_33KHz
+                ChannelSpacing50KHz,  //!< 50kHz
+                ChannelSpacing25KHz,  //!< 25kHz
+                ChannelSpacing8_33KHz //!< 8.33kHz
             };
 
             //! COM unit