diff --git a/python/obitools3/format/tab.pyx b/python/obitools3/format/tab.pyx
index d05b4c9..7af45df 100755
--- a/python/obitools3/format/tab.pyx
+++ b/python/obitools3/format/tab.pyx
@@ -23,16 +23,17 @@ cdef class TabFormat:
     @cython.boundscheck(False)    
     def __call__(self, object data):
         
-        cdef set ktags
+        cdef object ktags
         cdef list tags = [key for key in data]
         
         line = []
-        
-        if self.tags is not None and self.tags:
-            ktags = self.tags
+        if self.tags != None and self.tags:
+            ktags = list(self.tags)
         else:
-            ktags = set(tags)            
-                
+            ktags = list(set(tags))   
+                     
+        ktags.sort()
+                        
         if self.header and self.first_line:
             for k in ktags:
                 if k in tags: