diff --git a/src/parallel.h b/src/parallel.h
index 4a6daa8..cca4164 100644
--- a/src/parallel.h
+++ b/src/parallel.h
@@ -2,7 +2,6 @@
 
 #include <functional>
 #include "ProgressBar.h"
-#include <boost/optional/optional.hpp>
 #include <gsl_util.h>
 
 template<typename TCollection>
@@ -11,6 +10,18 @@ void runParallel(
 	TCollection& collection,
 	int maxThreadCount)
 {
+	if (maxThreadCount < 1) {
+		throw std::invalid_argument(fmt::format("maxThreadCount cannot be {}.", maxThreadCount));
+	}
+
+	if (maxThreadCount == 1) {
+		// Process synchronously
+		for (auto& element : collection) {
+			processElement(element);
+		}
+		return;
+	}
+
 	using future_type = std::future<void>;
 
 	std::mutex mutex;
diff --git a/src/phoneExtraction.cpp b/src/phoneExtraction.cpp
index a40dadd..e028e57 100644
--- a/src/phoneExtraction.cpp
+++ b/src/phoneExtraction.cpp
@@ -444,7 +444,10 @@ BoundedTimeline<Phone> detectPhones(
 			// Don't waste time creating additional threads (and decoders!) if the recording is short
 			static_cast<int>(duration_cast<std::chrono::seconds>(audioClip->getTruncatedRange().getLength()).count() / 5)
 		});
-		logging::debug("Speech recognition -- start");
+		if (threadCount < 1) {
+			threadCount = 1;
+		}
+		logging::debugFormat("Speech recognition using {} threads -- start", threadCount);
 		runParallel(processUtterance, utterances, threadCount, dialogProgressSink, getUtteranceProgressWeight);
 		logging::debug("Speech recognition -- end");
 	}