diff --git a/toolkit.py b/toolkit.py
index 341a768..6ee1cc7 100644
--- a/toolkit.py
+++ b/toolkit.py
@@ -691,7 +691,7 @@ def write_trace_header(fh, trace_header, trace_header_format, pos=None):
     """
     if pos is not None:
         fh.seek(pos, os.SEEK_SET)
-    buf = trace_header_format.pack(trace_header)
+    buf = trace_header_format.pack(*trace_header)
     fh.write(buf)
 
 
@@ -827,4 +827,4 @@ if __name__ == '__main__':
                                             sweep_length_ms=10000,
                                             sweep_channel_number=3,
                                             sweep_type='spread')
-    pp(header, width=200)
\ No newline at end of file
+    pp(header, width=200)