diff --git a/xrspatial/geotiff/__init__.py b/xrspatial/geotiff/__init__.py
index e1c29f22..8f3d0de2 100644
--- a/xrspatial/geotiff/__init__.py
+++ b/xrspatial/geotiff/__init__.py
@@ -2459,13 +2459,20 @@ def read_geotiff_dask(source: str, *,
     # oversized request before any task is scheduled saves the caller
     # from a misleading "tile size exceeds max_pixels" error in a
     # chunk that happens to align with the file's tile grid.
-    if max_pixels is not None:
-        eff_bands = (1 if band is not None
-                     else (n_bands if n_bands > 0 else 1))
-        if full_h * full_w * eff_bands > max_pixels:
-            raise ValueError(
-                f"Requested region {full_h}x{full_w}x{eff_bands} "
-                f"exceeds max_pixels={max_pixels:,}.")
+    # ``max_pixels=None`` substitutes the module default to match the
+    # eager (``read_to_array``) and VRT chunked paths. Without the
+    # substitution the guard would skip entirely on ``None`` and a
+    # caller could build a lazy graph over a region far larger than the
+    # documented safety cap. See issue #1838.
+    from ._reader import MAX_PIXELS_DEFAULT as _MAX_PIXELS_DEFAULT
+    effective_max_pixels = (max_pixels if max_pixels is not None
+                            else _MAX_PIXELS_DEFAULT)
+    eff_bands = (1 if band is not None
+                 else (n_bands if n_bands > 0 else 1))
+    if full_h * full_w * eff_bands > effective_max_pixels:
+        raise ValueError(
+            f"Requested region {full_h}x{full_w}x{eff_bands} "
+            f"exceeds max_pixels={effective_max_pixels:,}.")
 
     if name is None:
         import os
diff --git a/xrspatial/geotiff/tests/test_dask_max_pixels_default_guard_1838.py b/xrspatial/geotiff/tests/test_dask_max_pixels_default_guard_1838.py
new file mode 100644
index 00000000..6326d905
--- /dev/null
+++ b/xrspatial/geotiff/tests/test_dask_max_pixels_default_guard_1838.py
@@ -0,0 +1,85 @@
+"""``read_geotiff_dask(max_pixels=None)`` must honour the module default.
+
+The eager (``read_to_array``) and VRT chunked paths both substitute
+``MAX_PIXELS_DEFAULT`` for ``None`` before applying the up-front pixel
+count guard. ``read_geotiff_dask`` previously gated the guard on
+``max_pixels is not None``, so callers could build a lazy graph over a
+region far larger than the module-wide safety limit -- individual chunk
+reads still fail at compute time, but the cheap up-front error path was
+skipped, contradicting the documented "None uses the default cap"
+semantics. Issue #1838.
+"""
+from __future__ import annotations
+
+import numpy as np
+import pytest
+
+tifffile = pytest.importorskip("tifffile")
+
+from xrspatial.geotiff import read_geotiff_dask
+from xrspatial.geotiff._reader import MAX_PIXELS_DEFAULT
+
+
+def _write_oversized(path, *, h: int, w: int) -> None:
+    """Write a tiny tiled TIFF whose declared dimensions exceed the cap.
+
+    ``tifffile`` will not let us materialise a multi-billion-pixel array,
+    so we exploit the fact that the dask reader only consults the
+    header's ImageLength / ImageWidth tags for the up-front guard. The
+    physical file is one tile; the header advertises a much larger
+    image. Reading any window-less chunk would fail at decode time, but
+    that is acceptable because the up-front guard is supposed to fire
+    long before chunk tasks run.
+    """
+    # Smallest possible single-tile file; declare a tiny image so the
+    # file is valid, then patch the IFD's width/length tags to advertise
+    # an oversized image.
+    arr = np.zeros((16, 16), dtype=np.uint8)
+    tifffile.imwrite(str(path), arr, tile=(16, 16),
+                     photometric="minisblack", compression="none")
+    # Rewrite the ImageWidth (256) and ImageLength (257) tags in the IFD.
+    # tifffile writes a classic TIFF; the IFD starts at offset 8 for a
+    # small file. Parsing the offset properly requires reading the
+    # header; do that with tifffile's own parser to stay robust.
+    with tifffile.TiffFile(str(path)) as tf:
+        page = tf.pages[0]
+        ifd_offset = page.offset
+    raw = bytearray(path.read_bytes())
+    # Little-endian classic TIFF; first 2 bytes of IFD = entry count.
+    n_entries = int.from_bytes(raw[ifd_offset:ifd_offset + 2], 'little')
+    for i in range(n_entries):
+        entry = ifd_offset + 2 + i * 12
+        tag = int.from_bytes(raw[entry:entry + 2], 'little')
+        if tag == 256:  # ImageWidth
+            raw[entry + 8:entry + 12] = int(w).to_bytes(4, 'little')
+        elif tag == 257:  # ImageLength
+            raw[entry + 8:entry + 12] = int(h).to_bytes(4, 'little')
+    path.write_bytes(bytes(raw))
+
+
+def test_default_max_pixels_guard_fires_for_full_region(tmp_path):
+    """``max_pixels=None`` must apply the module default cap at the
+    up-front region guard, matching the eager / VRT paths.
+    """
+    path = tmp_path / "tmp_1838_oversized.tif"
+    side = int((MAX_PIXELS_DEFAULT ** 0.5)) + 2
+    _write_oversized(path, h=side, w=side)
+    with pytest.raises(ValueError, match=r"max_pixels"):
+        read_geotiff_dask(str(path))
+
+
+def test_explicit_max_pixels_still_enforced(tmp_path):
+    path = tmp_path / "tmp_1838_explicit_cap.tif"
+    _write_oversized(path, h=2048, w=2048)
+    with pytest.raises(ValueError, match=r"max_pixels"):
+        read_geotiff_dask(str(path), max_pixels=1024)
+
+
+def test_small_region_unaffected(tmp_path):
+    """The default cap must not interfere with normal small reads."""
+    arr = np.arange(64, dtype=np.uint8).reshape(8, 8)
+    path = tmp_path / "tmp_1838_small.tif"
+    tifffile.imwrite(str(path), arr, tile=(16, 16),
+                     photometric="minisblack", compression="none")
+    da = read_geotiff_dask(str(path), chunks=8)
+    np.testing.assert_array_equal(da.compute().values, arr)