diff --git a/gambatte_qt/src/framework/src/sourceupdater.cpp b/gambatte_qt/src/framework/src/sourceupdater.cpp
index ffd2eefb..918c8e93 100644
--- a/gambatte_qt/src/framework/src/sourceupdater.cpp
+++ b/gambatte_qt/src/framework/src/sourceupdater.cpp
@@ -45,7 +45,7 @@ std::ptrdiff_t SourceUpdater::update(PixelBuffer const &pb) {
 		               updateSamples);
 	samplesBuffered_ += updateSamples;
 	return vidFrameDoneSampleNo >= 0
-	     ? samplesBuffered_ - updateSamples + vidFrameDoneSampleNo
+	     ? std::ptrdiff_t(samplesBuffered_ - updateSamples + vidFrameDoneSampleNo)
 	     : -1;
 }