diff --git a/include/audio.hpp b/include/audio.hpp
index fc116da9..558b3125 100644
--- a/include/audio.hpp
+++ b/include/audio.hpp
@@ -73,9 +73,6 @@ Methods throw `rack::Exception` if the driver API has an exception.
 struct Device {
 	std::set<Port*> subscribed;
 	virtual ~Device() {}
-	// Called by Driver::subscribe().
-	void subscribe(Port* port);
-	void unsubscribe(Port* port);
 
 	virtual std::string getName() {
 		return "";
@@ -113,6 +110,10 @@ struct Device {
 	/** Sets the block size of the device, re-opening it if needed. */
 	virtual void setBlockSize(int blockSize) {}
 
+	// Called by Driver::subscribe().
+	virtual void subscribe(Port* port);
+	virtual void unsubscribe(Port* port);
+
 	// Called by this Device class, forwards to subscribed Ports.
 	void processBuffer(const float* input, int inputStride, float* output, int outputStride, int frames);
 	void onStartStream();
diff --git a/src/rtaudio.cpp b/src/rtaudio.cpp
index 4d3144a1..bcb08971 100644
--- a/src/rtaudio.cpp
+++ b/src/rtaudio.cpp
@@ -1,5 +1,6 @@
 #include <map>
 #include <algorithm>
+#include <mutex>
 
 #pragma GCC diagnostic push
 #ifndef __clang__
@@ -42,6 +43,8 @@ struct RtAudioDevice : audio::Device {
 	RtAudio::StreamOptions options;
 	int blockSize = 0;
 	float sampleRate = 0;
+	/** Ensures that ports do not subscribe/unsubscribe while processBuffer() is called. */
+	std::mutex processMutex;
 
 	RtAudioDevice(RtAudio::Api api, int deviceId) {
 		this->api = api;
@@ -209,18 +212,30 @@ struct RtAudioDevice : audio::Device {
 		openStream();
 	}
 
+	void subscribe(audio::Port* port) override {
+		std::lock_guard<std::mutex> lock(processMutex);
+		Device::subscribe(port);
+	}
+
+	void unsubscribe(audio::Port* port) override {
+		std::lock_guard<std::mutex> lock(processMutex);
+		Device::unsubscribe(port);
+	}
+
 	static int rtAudioCallback(void* outputBuffer, void* inputBuffer, unsigned int nFrames, double streamTime, RtAudioStreamStatus status, void* userData) {
 		// fprintf(stderr, ".");
 		// fflush(stderr);
 
-		RtAudioDevice* device = (RtAudioDevice*) userData;
-		assert(device);
+		RtAudioDevice* that = (RtAudioDevice*) userData;
+		assert(that);
+
+		std::lock_guard<std::mutex> lock(that->processMutex);
 
 		system::setThreadName("RtAudio");
 
-		int inputStride = device->getNumInputs();
-		int outputStride = device->getNumOutputs();
-		device->processBuffer((const float*) inputBuffer, inputStride, (float*) outputBuffer, outputStride, nFrames);
+		int inputStride = that->getNumInputs();
+		int outputStride = that->getNumOutputs();
+		that->processBuffer((const float*) inputBuffer, inputStride, (float*) outputBuffer, outputStride, nFrames);
 		return 0;
 	}
 };