--- a/dom/media/webaudio/AudioContext.h
+++ b/dom/media/webaudio/AudioContext.h
@@ -72,17 +72,17 @@ class Promise;
 enum class OscillatorType : uint32_t;
 
 // This is addrefed by the OscillatorNodeEngine on the main thread
 // and then used from the MSG thread.
 // It can be released either from the graph thread or the main thread.
 class BasicWaveFormCache
 {
 public:
-  BasicWaveFormCache(uint32_t aSampleRate);
+  explicit BasicWaveFormCache(uint32_t aSampleRate);
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(BasicWaveFormCache)
   WebCore::PeriodicWave* GetBasicWaveForm(OscillatorType aType);
 private:
   ~BasicWaveFormCache();
   nsRefPtr<WebCore::PeriodicWave> mSawtooth;
   nsRefPtr<WebCore::PeriodicWave> mSquare;
   nsRefPtr<WebCore::PeriodicWave> mTriangle;
   uint32_t mSampleRate;