diff --git a/Lib/fontTools/varLib/models.py b/Lib/fontTools/varLib/models.py
index 78086e8fc..6a9867c8a 100644
--- a/Lib/fontTools/varLib/models.py
+++ b/Lib/fontTools/varLib/models.py
@@ -61,13 +61,17 @@ def normalizeValue(v, triple, extrapolate=False):
         )
     if not extrapolate:
         v = max(min(v, upper), lower)
-    if v == default:
-        v = 0.0
-    elif v < default:
-        v = (v - default) / (default - lower)
+
+    if v == default or lower == upper:
+        return 0.0
+
+    if (v < default and lower != default) or (v > default and upper == default):
+        return (v - default) / (default - lower)
     else:
-        v = (v - default) / (upper - default)
-    return v
+        assert (v > default and upper != default) or (
+            v < default and lower == default
+        ), f"Ooops... v={v}, triple=({lower}, {default}, {upper})"
+        return (v - default) / (upper - default)
 
 
 def normalizeLocation(location, axes, extrapolate=False):
diff --git a/Tests/varLib/models_test.py b/Tests/varLib/models_test.py
index 58a047bb1..11ec1a1e8 100644
--- a/Tests/varLib/models_test.py
+++ b/Tests/varLib/models_test.py
@@ -31,19 +31,104 @@ def test_normalizeLocation():
     assert normalizeLocation({"wght": 1001}, axes) == {"wght": 0.0}
 
 
+@pytest.mark.parametrize(
+    "axes, location, expected",
+    [
+        # lower != default != upper
+        ({"wght": (100, 400, 900)}, {"wght": 1000}, {"wght": 1.2}),
+        ({"wght": (100, 400, 900)}, {"wght": 900}, {"wght": 1.0}),
+        ({"wght": (100, 400, 900)}, {"wght": 650}, {"wght": 0.5}),
+        ({"wght": (100, 400, 900)}, {"wght": 400}, {"wght": 0.0}),
+        ({"wght": (100, 400, 900)}, {"wght": 250}, {"wght": -0.5}),
+        ({"wght": (100, 400, 900)}, {"wght": 100}, {"wght": -1.0}),
+        ({"wght": (100, 400, 900)}, {"wght": 25}, {"wght": -1.25}),
+        # lower == default != upper
+        (
+            {"wght": (400, 400, 900), "wdth": (100, 100, 150)},
+            {"wght": 1000, "wdth": 200},
+            {"wght": 1.2, "wdth": 2.0},
+        ),
+        (
+            {"wght": (400, 400, 900), "wdth": (100, 100, 150)},
+            {"wght": 25, "wdth": 25},
+            {"wght": -0.75, "wdth": -1.5},
+        ),
+        # lower != default == upper
+        (
+            {"wght": (100, 400, 400), "wdth": (50, 100, 100)},
+            {"wght": 700, "wdth": 150},
+            {"wght": 1.0, "wdth": 1.0},
+        ),
+        (
+            {"wght": (100, 400, 400), "wdth": (50, 100, 100)},
+            {"wght": -50, "wdth": 25},
+            {"wght": -1.5, "wdth": -1.5},
+        ),
+        # degenerate case with lower == default == upper, normalized location always 0
+        ({"wght": (400, 400, 400)}, {"wght": 100}, {"wght": 0.0}),
+        ({"wght": (400, 400, 400)}, {"wght": 400}, {"wght": 0.0}),
+        ({"wght": (400, 400, 400)}, {"wght": 700}, {"wght": 0.0}),
+    ],
+)
+def test_normalizeLocation_extrapolate(axes, location, expected):
+    assert normalizeLocation(location, axes, extrapolate=True) == expected
+
+
 def test_supportScalar():
     assert supportScalar({}, {}) == 1.0
     assert supportScalar({"wght": 0.2}, {}) == 1.0
     assert supportScalar({"wght": 0.2}, {"wght": (0, 2, 3)}) == 0.1
     assert supportScalar({"wght": 2.5}, {"wght": (0, 2, 4)}) == 0.75
     assert supportScalar({"wght": 3}, {"wght": (0, 2, 2)}) == 0.0
-    assert supportScalar({"wght": 3}, {"wght": (0, 2, 2)}, extrapolate=True, axisRanges={"wght": (0, 2)}) == 1.5
-    assert supportScalar({"wght": -1}, {"wght": (0, 2, 2)}, extrapolate=True, axisRanges={"wght": (0, 2)}) == -0.5
-    assert supportScalar({"wght": 3}, {"wght": (0, 1, 2)}, extrapolate=True, axisRanges={"wght": (0, 2)}) == -1.0
-    assert supportScalar({"wght": -1}, {"wght": (0, 1, 2)}, extrapolate=True, axisRanges={"wght": (0, 2)}) == -1.0
-    assert supportScalar({"wght": 2}, {"wght": (0, 0.75, 1)}, extrapolate=True, axisRanges={"wght": (0, 1)}) == -4.0
+    assert (
+        supportScalar(
+            {"wght": 3},
+            {"wght": (0, 2, 2)},
+            extrapolate=True,
+            axisRanges={"wght": (0, 2)},
+        )
+        == 1.5
+    )
+    assert (
+        supportScalar(
+            {"wght": -1},
+            {"wght": (0, 2, 2)},
+            extrapolate=True,
+            axisRanges={"wght": (0, 2)},
+        )
+        == -0.5
+    )
+    assert (
+        supportScalar(
+            {"wght": 3},
+            {"wght": (0, 1, 2)},
+            extrapolate=True,
+            axisRanges={"wght": (0, 2)},
+        )
+        == -1.0
+    )
+    assert (
+        supportScalar(
+            {"wght": -1},
+            {"wght": (0, 1, 2)},
+            extrapolate=True,
+            axisRanges={"wght": (0, 2)},
+        )
+        == -1.0
+    )
+    assert (
+        supportScalar(
+            {"wght": 2},
+            {"wght": (0, 0.75, 1)},
+            extrapolate=True,
+            axisRanges={"wght": (0, 1)},
+        )
+        == -4.0
+    )
     with pytest.raises(TypeError):
-        supportScalar({"wght": 2}, {"wght": (0, 0.75, 1)}, extrapolate=True, axisRanges=None)
+        supportScalar(
+            {"wght": 2}, {"wght": (0, 0.75, 1)}, extrapolate=True, axisRanges=None
+        )
 
 
 def test_model_extrapolate():