diff --git a/python/obitools3/commands/ngsfilter.pyx b/python/obitools3/commands/ngsfilter.pyx
old mode 100644
new mode 100755
index 131201c..e7356e9
--- a/python/obitools3/commands/ngsfilter.pyx
+++ b/python/obitools3/commands/ngsfilter.pyx
@@ -271,7 +271,7 @@ cdef tuple annotate(sequences, infos, no_tags, verbose=False):
         sequences[0][REVERSE_QUALITY_COLUMN] = sequences[1].quality     # used by alignpairedend tool
 
     for seq in sequences:
-        if hasattr(seq, "quality_array"): 
+        if hasattr(seq, "quality_array") and seq.quality_array is not None: 
             q = -reduce(lambda x,y:x+y,(math.log10(z) for z in seq.quality_array),0)/len(seq.quality_array)*10
             seq[b'avg_quality']=q
             q = -reduce(lambda x,y:x+y,(math.log10(z) for z in seq.quality_array[0:10]),0)