diff --git a/conversion.go b/conversion.go
index 405d26d..993d870 100644
--- a/conversion.go
+++ b/conversion.go
@@ -17,12 +17,12 @@ func makeConcatDemuxFile(manifest UgoiraManifest, frameDescFile *os.File) {
 	frames := manifest.Body.Frames
 	for _, frame := range frames {
 		frameDur := float32(frame.Delay) / 1000 // convert to seconds
-		frameDescFile.WriteString(fmt.Sprintf("file '%s'\n", frame.File))
-		frameDescFile.WriteString(fmt.Sprintf("duration %f\n", frameDur))
+		fmt.Fprintf(frameDescFile, "file '%s'\n", frame.File)
+		fmt.Fprintf(frameDescFile, "duration %f\n", frameDur)
 	}
 	// repeat last frame (see: https://trac.ffmpeg.org/wiki/Slideshow#Concatdemuxer
 	lastFrame := frames[len(frames)-1]
-	frameDescFile.WriteString(fmt.Sprintf("file '%s'\n", lastFrame.File))
+	fmt.Fprintf(frameDescFile, "file '%s'\n", lastFrame.File)
 }
 
 func callFFmpeg(frameDescPath, outputFilePath, workDir, ffmpegArgs string) error {