diff --git a/augur/ancestral.py b/augur/ancestral.py
index 8c66041e1..3ee0e2828 100644
--- a/augur/ancestral.py
+++ b/augur/ancestral.py
@@ -72,8 +72,7 @@ def ancestral_sequence_inference(tree=None, aln=None, ref=None, infer_gtr=True,
 
     return tt
 
-def collect_mutations_and_sequences(tt, infer_tips=False, full_sequences=False, character_map=None,
-                                    mask_ambiguous=True):
+def collect_mutations_and_sequences(tt, infer_tips=False, full_sequences=False, character_map=None, is_vcf=False):
     """iterates of the tree and produces dictionaries with
     mutations and sequences for each node.
 
@@ -104,20 +103,19 @@ def collect_mutations_and_sequences(tt, infer_tips=False, full_sequences=False,
         data[n.name]['muts'] = [a+str(int(pos)+inc)+cm(d)
                                 for a,pos,d in n.mutations]
 
-    mask=None
-    if full_sequences:
-        if mask_ambiguous:
-            # Identify sites for which every terminal sequence is ambiguous.
-            # These sites will be masked to prevent rounding errors in the
-            # maximum likelihood inference from assigning an arbitrary
-            # nucleotide to sites at internal nodes.
-            ambiguous_count = np.zeros(tt.sequence_length, dtype=int)
-            for n in tt.tree.get_terminals():
-                ambiguous_count += np.array(tt.sequence(n,reconstructed=False, as_string=False)==tt.gtr.ambiguous, dtype=int)
-            mask = ambiguous_count==len(tt.tree.get_terminals())
-        else:
-            mask = np.zeros(tt.sequence_length, dtype=bool)
+    if is_vcf:
+        mask = np.zeros(tt.sequence_length, dtype=bool)
+    else:
+        # Identify sites for which every terminal sequence is ambiguous.
+        # These sites will be masked to prevent rounding errors in the
+        # maximum likelihood inference from assigning an arbitrary
+        # nucleotide to sites at internal nodes.
+        ambiguous_count = np.zeros(tt.sequence_length, dtype=int)
+        for n in tt.tree.get_terminals():
+            ambiguous_count += np.array(tt.sequence(n,reconstructed=False, as_string=False)==tt.gtr.ambiguous, dtype=int)
+        mask = ambiguous_count==len(tt.tree.get_terminals())
 
+    if full_sequences:
         for n in tt.tree.find_clades():
             try:
                 tmp = tt.sequence(n,reconstructed=infer_tips, as_string=False)
@@ -150,7 +148,7 @@ def run_ancestral(T, aln, root_sequence=None, is_vcf=False, full_sequences=False
         root_seq = tt.sequence(T.root, as_string=True)
 
     mutations = collect_mutations_and_sequences(tt, full_sequences=full_sequences,
-                          infer_tips=infer_ambiguous, character_map=character_map)
+                          infer_tips=infer_ambiguous, character_map=character_map, is_vcf=is_vcf)
     if root_sequence:
         for pos, (root_state, tree_state) in enumerate(zip(root_sequence, tt.sequence(tt.tree.root, reconstructed=infer_ambiguous, as_string=True))):
             if root_state != tree_state:
@@ -249,9 +247,9 @@ def run(args):
     # explicitly or by default) and the user has not explicitly requested that
     # we keep them.
     infer_ambiguous = args.infer_ambiguous and not args.keep_ambiguous
-
+    full_sequences = args.output_sequences is not None
     nuc_result = run_ancestral(T, aln, root_sequence=str(ref.seq) if ref else None, is_vcf=is_vcf, fill_overhangs=not args.keep_overhangs,
-                               marginal=args.inference, infer_ambiguous=infer_ambiguous, alphabet='nuc')
+                               full_sequences=full_sequences, marginal=args.inference, infer_ambiguous=infer_ambiguous, alphabet='nuc')
     anc_seqs = nuc_result['mutations']
     anc_seqs['reference'] = {'nuc': nuc_result['root_seq']}