diff --git a/openpilot/selfdrive/locationd/helpers.py b/openpilot/selfdrive/locationd/helpers.py
index fe7930c509f7d1..adcc7493a9ec4b 100644
--- a/openpilot/selfdrive/locationd/helpers.py
+++ b/openpilot/selfdrive/locationd/helpers.py
@@ -34,7 +34,7 @@ def fft_next_good_size(n: int) -> int:
 
 
 def parabolic_peak_interp(R, max_index):
-  if max_index == 0 or max_index == len(R) - 1:
+  if max_index <= 0 or max_index >= len(R) - 1:
     return max_index
 
   y_m1, y_0, y_p1 = R[max_index - 1], R[max_index], R[max_index + 1]
diff --git a/openpilot/selfdrive/locationd/test/test_helpers.py b/openpilot/selfdrive/locationd/test/test_helpers.py
new file mode 100644
index 00000000000000..60121439e55851
--- /dev/null
+++ b/openpilot/selfdrive/locationd/test/test_helpers.py
@@ -0,0 +1,16 @@
+from openpilot.selfdrive.locationd.helpers import parabolic_peak_interp
+
+
+class TestParabolicPeakInterp:
+  def test_symmetric_peak_refines_to_center(self):
+    assert parabolic_peak_interp([0.0, 1.0, 0.0], 1) == 1.0
+
+  def test_endpoints_return_index(self):
+    R = [1.0, 0.5, 0.2]
+    assert parabolic_peak_interp(R, 0) == 0
+    assert parabolic_peak_interp(R, len(R) - 1) == len(R) - 1
+
+  def test_degenerate_inputs_do_not_raise(self):
+    # an empty array or an out-of-range index must not raise IndexError
+    assert parabolic_peak_interp([], 7) == 7
+    assert parabolic_peak_interp([1.0, 2.0], 5) == 5