diff --git a/src/blackmisc/aviation/altitude.cpp b/src/blackmisc/aviation/altitude.cpp
index 82ef5db4f..6c9788375 100644
--- a/src/blackmisc/aviation/altitude.cpp
+++ b/src/blackmisc/aviation/altitude.cpp
@@ -88,16 +88,18 @@ namespace BlackMisc
             }
         }
 
-        void CAltitude::toFlightLevel()
+        bool CAltitude::toFlightLevel()
         {
-            Q_ASSERT(m_datum == MeanSeaLevel || m_datum == FlightLevel);
+            if (m_datum != MeanSeaLevel && m_datum != FlightLevel) { return false; }
             m_datum = FlightLevel;
+            return true;
         }
 
-        void CAltitude::toMeanSeaLevel()
+        bool CAltitude::toMeanSeaLevel()
         {
-            Q_ASSERT(m_datum == MeanSeaLevel || m_datum == FlightLevel);
+            if (m_datum != MeanSeaLevel && m_datum != FlightLevel) { return false; }
             m_datum = MeanSeaLevel;
+            return true;
         }
 
         void CAltitude::convertToPressureAltitude(const CPressure &seaLevelPressure)
@@ -173,7 +175,7 @@ namespace BlackMisc
         {
             QString v(value.trimmed());
             this->setNull();
-            if (v.isEmpty() || v.length() < 3)
+            if (v.length() < 3)
             {
                 if (msgs) { msgs->push_back(CStatusMessage(this).validationError("Altitude empty or too short")); }
                 return false;
diff --git a/src/blackmisc/aviation/altitude.h b/src/blackmisc/aviation/altitude.h
index ea18edd52..9b3d01b15 100644
--- a/src/blackmisc/aviation/altitude.h
+++ b/src/blackmisc/aviation/altitude.h
@@ -133,10 +133,10 @@ namespace BlackMisc
             ReferenceDatum getReferenceDatum() const { return m_datum; }
 
             //! MSL to flightlevel
-            void toFlightLevel();
+            bool toFlightLevel();
 
             //! Flightlevel to MSL
-            void toMeanSeaLevel();
+            bool toMeanSeaLevel();
 
             //! Current altitude type
             AltitudeType getAltitudeType() const { return m_altitudeType; }