diff --git a/scripts/compare_host_align.py b/scripts/compare_host_align.py
index 873d2f9..8bf6017 100644
--- a/scripts/compare_host_align.py
+++ b/scripts/compare_host_align.py
@@ -10,7 +10,10 @@ pixel, how the two aligned depths differ. Prints one JSON line per frameset
 and a summary. Only GET requests; nothing on PC2 changes.
 
 The host's pyrealsense2 may be newer than PC2's (2.58 here, 2.50 there): the
-two aligns are then not even the same code version.
+two aligns are then not even the same code version. It must have
+software_video_frame.depth_units: pyrealsense2 2.50 has not, and there the
+software frames' depth scale is 0, so rs.align returns an (almost) empty
+depth WITHOUT any error. The script refuses to run on such a version.
 """
 
 import argparse
@@ -51,10 +54,19 @@ def rs_extrinsics(ext: dict) -> rs.extrinsics:
 
 # Host align ---------------------------------------------------------------------
 
+def require_software_depth_units() -> None:
+    """Raise unless this pyrealsense2 can give software depth frames their units (not 2.50)."""
+    if not hasattr(rs.software_video_frame(), "depth_units"):
+        raise RuntimeError(
+            f"pyrealsense2 {getattr(rs, '__version__', '(no __version__, e.g. 2.50)')} has no "
+            "software_video_frame.depth_units: the host align would come out empty. Use a newer one (2.58 works).")
+
+
 class HostAligner:
     """rs.align(color) on frames pushed into a software_device with PC2's calibration."""
 
     def __init__(self, calibration: dict):
+        require_software_depth_units()
         self._device = rs.software_device()
         self._depth = self._device.add_sensor("Depth")
         self._color = self._device.add_sensor("Color")
@@ -129,7 +141,7 @@ def _push(sensor, profile, pixels: np.ndarray, bpp: int, number: int, stamp_ms:
     frame.frame_number = number
     frame.profile = profile.as_video_stream_profile()
     if depth:
-        frame.depth_units = 0.001
+        frame.depth_units = 0.001   # millimeters (checked to exist by require_software_depth_units)
     sensor.on_video_frame(frame)
 
 
@@ -204,7 +216,7 @@ def main(argv=None) -> int:
                 results.append(result)
                 print(json.dumps(result))
     means = [r["abs_diff_mm"]["mean"] for r in results if r["abs_diff_mm"]]
-    print(json.dumps({"summary": {"framesets": len(results), "pyrealsense2_host": rs.__version__,
+    print(json.dumps({"summary": {"framesets": len(results), "pyrealsense2_host": getattr(rs, "__version__", "unknown"),
                                   "mean_abs_diff_mm_per_frameset": means}}))
     return 0