diff --git a/pytta/classes/_instanceinfo.py b/pytta/classes/_instanceinfo.py
index ab2a11c..5e4aad2 100644
--- a/pytta/classes/_instanceinfo.py
+++ b/pytta/classes/_instanceinfo.py
@@ -28,7 +28,8 @@ def __init__(self):
             varnames = frame.f_code.co_varnames
             if varnames == ():
                 break
-            if frame.f_locals[varnames[0]] not in (self, self.__class__):
+            first = frame.f_locals.get(varnames[0])
+            if first is not self and first is not self.__class__:
                 break
                 # if the frame is inside a method of this instance,
                 # the first argument usually contains either the instance or
diff --git a/tests/test_signalobj_creation.py b/tests/test_signalobj_creation.py
new file mode 100644
index 0000000..e07cee7
--- /dev/null
+++ b/tests/test_signalobj_creation.py
@@ -0,0 +1,40 @@
+"""
+Tests for the creation of a SignalObj from inside a function.
+
+SignalObj reads the name of the variable it is assigned to from the call
+stack (classes/_instanceinfo.py). The lookup must not depend on what the
+calling function holds in its first local variable. Each test pins one
+case that used to raise.
+"""
+import os
+import unittest
+
+os.environ.setdefault('MPLBACKEND', 'Agg')
+
+import numpy as np
+
+import pytta
+
+
+def from_array(data):
+    # first local variable: a NumPy array
+    return pytta.SignalObj(data, 'time', 48000)
+
+
+def before_first_local_is_set():
+    # first local variable not assigned yet when SignalObj is created
+    sig = pytta.SignalObj(np.ones(8), 'time', 48000)
+    return sig
+
+
+class TestSignalObjCreation(unittest.TestCase):
+
+    def test_caller_holding_an_array(self):
+        self.assertEqual(from_array(np.ones(8)).numSamples, 8)
+
+    def test_caller_whose_first_local_is_not_set(self):
+        self.assertEqual(before_first_local_is_set().numSamples, 8)
+
+
+if __name__ == '__main__':
+    unittest.main()