diff --git a/terracotta/server/fields.py b/terracotta/server/fields.py
index defbd08b..af758299 100644
--- a/terracotta/server/fields.py
+++ b/terracotta/server/fields.py
@@ -27,11 +27,8 @@ def _deserialize(
 
 
 def validate_stretch_range(data: Any) -> None:
-    if isinstance(data, str) and data.startswith("p"):
+    if isinstance(data, str):
         if not re.match("^p\\d+$", data):
             raise ValidationError("Percentile format is `p<digits>`")
-    else:
-        try:
-            float(data)
-        except ValueError:
-            raise ValidationError("Must be a number")
+    elif not isinstance(data, (int, float)):
+        raise ValidationError("Must be a number or string like `p<digits>`")
diff --git a/tests/handlers/test_rgb.py b/tests/handlers/test_rgb.py
index 07a59eee..358c4f85 100644
--- a/tests/handlers/test_rgb.py
+++ b/tests/handlers/test_rgb.py
@@ -3,6 +3,8 @@
 
 import pytest
 
+from terracotta import exceptions
+
 
 def test_rgb_handler(use_testdb, raster_file, raster_file_xyz):
     import terracotta
@@ -184,6 +186,35 @@ def test_rgb_percentile_stretch(use_testdb, testdb, raster_file_xyz):
     assert np.all(valid_img[valid_data > stretch_range[1]] == 255)
 
 
+@pytest.mark.parametrize(
+    "stretch_range_params",
+    [
+        ["s2", "p98", "Invalid scale value"],
+        ["pp2", "p98", "Invalid percentile value"],
+        ["p", "p98", "Invalid percentile value"],
+        ["2", "p8", "Invalid scale value"],
+        [{}, "p98", "Invalid scale value"],
+        ["p-2", "p98", "Invalid percentile, out of range"],
+        ["p2", "p298", "Invalid percentile, out of range"],
+    ],
+)
+def test_rgb_invalid_percentiles(use_testdb, raster_file_xyz, stretch_range_params):
+    from terracotta.handlers import rgb
+
+    ds_keys = ["val21", "x", "val22"]
+    bands = ["val22", "val23", "val24"]
+
+    stretch_range = stretch_range_params[:2]
+    with pytest.raises(exceptions.InvalidArgumentsError) as err:
+        rgb.rgb(
+            ds_keys[:2],
+            bands,
+            raster_file_xyz,
+            stretch_ranges=[stretch_range] * 3,
+        )
+    assert stretch_range_params[2] in str(err.value)
+
+
 def test_rgb_preview(use_testdb):
     import terracotta
     from terracotta.handlers import rgb
diff --git a/tests/handlers/test_singleband.py b/tests/handlers/test_singleband.py
index 09dbf51f..7029737a 100644
--- a/tests/handlers/test_singleband.py
+++ b/tests/handlers/test_singleband.py
@@ -3,6 +3,8 @@
 
 import pytest
 
+from terracotta import exceptions
+
 
 @pytest.mark.parametrize("resampling_method", ["nearest", "linear", "cubic", "average"])
 def test_singleband_handler(use_testdb, raster_file_xyz, resampling_method):
@@ -196,3 +198,30 @@ def test_singleband_stretch_percentile(use_testdb, testdb, raster_file_xyz):
     assert np.all(valid_img[stretch_range_mask] >= 1)
     assert np.all(valid_img[stretch_range_mask] <= 255)
     assert np.all(valid_img[valid_data > stretch_range[1]] == 255)
+
+
+@pytest.mark.parametrize(
+    "stretch_range_params",
+    [
+        ["s2", "p98", "Invalid scale value"],
+        ["pp2", "p98", "Invalid percentile value"],
+        ["p", "p98", "Invalid percentile value"],
+        ["2", "p8", "Invalid scale value"],
+        [{}, "p98", "Invalid scale value"],
+        ["p-2", "p98", "Invalid percentile, out of range"],
+        ["p2", "p298", "Invalid percentile, out of range"],
+    ],
+)
+def test_rgb_invalid_percentiles(use_testdb, stretch_range_params):
+    from terracotta.handlers import singleband
+
+    ds_keys = ["val21", "x", "val22"]
+
+    stretch_range = stretch_range_params[:2]
+
+    with pytest.raises(exceptions.InvalidArgumentsError) as err:
+        singleband.singleband(
+            ds_keys,
+            stretch_range=stretch_range,
+        )
+    assert stretch_range_params[2] in str(err.value)
diff --git a/tests/server/test_flask_api.py b/tests/server/test_flask_api.py
index e153c2f0..21a107ff 100644
--- a/tests/server/test_flask_api.py
+++ b/tests/server/test_flask_api.py
@@ -3,6 +3,7 @@
 import urllib.parse
 
 from PIL import Image
+import marshmallow
 import numpy as np
 
 import pytest
@@ -562,3 +563,27 @@ def test_get_spec(client):
     rv = client.get("/apidoc")
     assert rv.status_code == 200
     assert b"Terracotta" in rv.data
+
+
+@pytest.mark.parametrize(
+    "stretch_range_params",
+    [
+        ['["s2", "p98"]', "Percentile format is"],
+        ['["pp2", "p98"]', "Percentile format is"],
+        ['["p", "p98"]', "Percentile format is"],
+        ['["2", "p98"]', "Percentile format is"],
+        ['["[]", "p98"]', "Percentile format is"],
+    ],
+)
+def test_get_rgb_invalid_stretch_percentile(
+    debug_client, use_testdb, raster_file_xyz, stretch_range_params
+):
+
+    x, y, z = raster_file_xyz
+    stretch_range = stretch_range_params[0]
+    with pytest.raises(marshmallow.ValidationError) as ve:
+        debug_client.get(
+            f"/rgb/val21/x/{z}/{x}/{y}.png?r=val22&g=val23&b=val24&"
+            f"r_range={stretch_range}&b_range={stretch_range}&g_range={stretch_range}"
+        )
+    assert stretch_range_params[1] in str(ve.value)