diff --git a/icartt/dataset.py b/icartt/dataset.py
index c25b7c35faf1adf60955413fb31210a17e11b9a0..bfa7e7549ea40f5d8f9b07bf2062c9973fd54785 100644
--- a/icartt/dataset.py
+++ b/icartt/dataset.py
@@ -78,7 +78,7 @@ class Dataset:
         # Mission name (usually the mission acronym).
         prnt(self.mission)
         # File volume number, number of file volumes (these integer values are used when the data require more than one file per day; for data that require only one file these values are set to 1, 1) - comma delimited.
-        prnt(self.splitChar.join([ self.VOL, self.NVOL ]))
+        prnt(self.splitChar.join([ str(self.VOL), str(self.NVOL) ]))
         # UTC date when data begin, UTC date of data reduction or revision - comma delimited (yyyy, mm, dd, yyyy, mm, dd).
         prnt(self.splitChar.join([ datetime.datetime.strftime(x, "%Y, %m, %d") for x in [ self.dateValid, self.dateRevised ] ]))
         # Data Interval (This value describes the time spacing (in seconds) between consecutive data records. It is the (constant) interval between values of the independent variable. For 1 Hz data the data interval value is 1 and for 10 Hz data the value is 0.1. All intervals longer than 1 second must be reported as Start and Stop times, and the Data Interval value is set to 0. The Mid-point time is required when it is not at the average of Start and Stop times. For additional information see Section 2.5 below.).