diff --git a/cmd/pnml.go b/cmd/pnml.go
index dfa891f..380c465 100644
--- a/cmd/pnml.go
+++ b/cmd/pnml.go
@@ -83,11 +83,13 @@ func pnmlConvert(filename string) {
 		return
 	}
 
-	// compute name of the output file. There are three possible cases
+	// compute name of the output file. There are three possible cases. We try
+	// to avoid overwriting over the input file by adding suffix -PT when not
+	// using option -o.
 	_, file := filepath.Split(filename)
 	outfile := pnmlOutFileName
 	if outfile == "" {
-		outfile = file[0 : len(file)-len(".pnml")]
+		outfile = file[0:len(file)-len(".pnml")] + "-PT"
 	}
 	if pnmlUseName {
 		outfile = p.Name
@@ -111,7 +113,7 @@ func pnmlConvert(filename string) {
 	} else {
 		out, err = os.Create(outfile + ".pnml")
 		if err != nil {
-			hlnetLogger.Println("Error creating result file: ", err, " (you may be trying to overwrite your input file!)")
+			hlnetLogger.Println("Error creating result file: ", err)
 			os.Exit(1)
 			return
 		}
diff --git a/corenet/pnmlwrite.go b/corenet/pnmlwrite.go
index 8da4f58..965340b 100644
--- a/corenet/pnmlwrite.go
+++ b/corenet/pnmlwrite.go
@@ -20,7 +20,7 @@ const (
 	DOCTYPE = `<?xml version="1.0" encoding="UTF-8"?>` + "\n"
 )
 
-// wpnml is the type of PNML for a P/T net ithout graphical information
+// wpnml is the type of PNML for a P/T net without graphical information
 type wpnml struct {
 	XMLName xml.Name `xml:"http://www.pnml.org/version-2009/grammar/pnml pnml"`
 	WNET    wnet     `xml:"net"`