diff --git a/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp b/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp
index 57368d827d..8b06ecb2e4 100644
--- a/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp
+++ b/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp
@@ -693,7 +693,9 @@ int64 AudioThumbnail::getHashCode() const
 void AudioThumbnail::addBlock (const int64 startSample, const AudioSampleBuffer& incoming,
                                int startOffsetInBuffer, int numSamples)
 {
-    jassert (startSample >= 0);
+    jassert (startSample >= 0
+              && startOffsetInBuffer >= 0
+              && startOffsetInBuffer + numSamples <= incoming.getNumSamples());
 
     const int firstThumbIndex = (int) (startSample / samplesPerThumbSample);
     const int lastThumbIndex  = (int) ((startSample + numSamples + (samplesPerThumbSample - 1)) / samplesPerThumbSample);