diff --git a/modules/juce_audio_devices/midi_io/juce_MidiMessageCollector.cpp b/modules/juce_audio_devices/midi_io/juce_MidiMessageCollector.cpp
index 4bcc897dca..8474a9301c 100644
--- a/modules/juce_audio_devices/midi_io/juce_MidiMessageCollector.cpp
+++ b/modules/juce_audio_devices/midi_io/juce_MidiMessageCollector.cpp
@@ -71,6 +71,7 @@ void MidiMessageCollector::removeNextBlockOfMessages (MidiBuffer& destBuffer,
 {
     // you need to call reset() to set the correct sample rate before using this object
     jassert (sampleRate != 44100.0001);
+    jassert (numSamples > 0);
 
     const double timeNow = Time::getMillisecondCounterHiRes();
     const double msElapsed = timeNow - lastCallbackTime;
diff --git a/modules/juce_audio_devices/midi_io/juce_MidiMessageCollector.h b/modules/juce_audio_devices/midi_io/juce_MidiMessageCollector.h
index 4530d670b3..c7442232c1 100644
--- a/modules/juce_audio_devices/midi_io/juce_MidiMessageCollector.h
+++ b/modules/juce_audio_devices/midi_io/juce_MidiMessageCollector.h
@@ -79,6 +79,8 @@ public:
 
         This method is fully thread-safe when overlapping calls are made with
         addMessageToQueue().
+
+        Precondition: numSamples must be greater than 0.
     */
     void removeNextBlockOfMessages (MidiBuffer& destBuffer, int numSamples);