diff --git a/nn_fingerprint.py b/nn_fingerprint.py
index 680fc23..709020a 100644
--- a/nn_fingerprint.py
+++ b/nn_fingerprint.py
@@ -115,7 +115,9 @@ class NNFingerprint(NNOperator):
             audio = numpy.vstack(frames).transpose()
         else:
             audio = numpy.hstack(frames)
-            audio = audio[None, :]
+            if len(audio.shape) == 1:
+                audio = audio[None, :]
+            assert len(audio.shape) == 2
         audio = self.int2float(audio)
 
         if sr != self.params['sample_rate']: