diff --git a/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp b/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp
index 88892bb394..dd9344bcf8 100644
--- a/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp
+++ b/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp
@@ -740,7 +740,7 @@ void AudioThumbnail::setLevels (const MinMaxValue* const* values, int thumbIndex
     if (numSamplesFinished >= start && end > numSamplesFinished)
         numSamplesFinished = end;
 
-    totalSamples = jmax (numSamplesFinished, totalSamples);
+    totalSamples = jmax (numSamplesFinished, totalSamples.load());
     window->invalidate();
     sendChangeMessage();
 }
@@ -763,7 +763,7 @@ bool AudioThumbnail::isFullyLoaded() const noexcept
 
 double AudioThumbnail::getProportionComplete() const noexcept
 {
-    return jlimit (0.0, 1.0, numSamplesFinished / (double) jmax ((int64) 1, totalSamples));
+    return jlimit (0.0, 1.0, numSamplesFinished / (double) jmax ((int64) 1, totalSamples.load()));
 }
 
 int64 AudioThumbnail::getNumSamplesFinished() const noexcept
diff --git a/modules/juce_audio_utils/gui/juce_AudioThumbnail.h b/modules/juce_audio_utils/gui/juce_AudioThumbnail.h
index 2496acb1d2..6dc133bb62 100644
--- a/modules/juce_audio_utils/gui/juce_AudioThumbnail.h
+++ b/modules/juce_audio_utils/gui/juce_AudioThumbnail.h
@@ -206,7 +206,8 @@ private:
     OwnedArray<ThumbData> channels;
 
     int32 samplesPerThumbSample = 0;
-    int64 totalSamples = 0, numSamplesFinished = 0;
+    std::atomic<int64> totalSamples { 0 };
+    int64 numSamplesFinished = 0;
     int32 numChannels = 0;
     double sampleRate = 0;
     CriticalSection lock;