diff --git a/xfel/command_line/detector_residuals.py b/xfel/command_line/detector_residuals.py
index 1724863d05..39065ef434 100644
--- a/xfel/command_line/detector_residuals.py
+++ b/xfel/command_line/detector_residuals.py
@@ -444,6 +444,14 @@ def run(self):
 
     experiments = flatten_experiments(params.input.experiments)
 
+    # XFEL stills .expt files carry a shared wavelength-less XFELBeam plus
+    # per-frame wavelengths in the scan; resolve each experiment's monochromatic
+    # beam up front so the residual/prediction code can call beam.get_s0().
+    # A no-op for ordinary beams. This tool only reads experiments, so it is
+    # safe to replace the beam in place here.
+    for expt in experiments:
+        expt.beam = expt.get_monochromatic_beam()
+
     # Find all detector objects
     detectors = experiments.detectors()
 
diff --git a/xfel/cxi/cspad_ana/mod_view.py b/xfel/cxi/cspad_ana/mod_view.py
index dcd2235fdc..1c8e48fe9e 100644
--- a/xfel/cxi/cspad_ana/mod_view.py
+++ b/xfel/cxi/cspad_ana/mod_view.py
@@ -62,7 +62,7 @@ def __init__(self, **kwargs):
 
     from copy import deepcopy
 
-    from dxtbx.model.beam import Beam, BeamFactory
+    from dxtbx.model.beam import BeamBase, BeamFactory
     from dxtbx.model.detector import Detector, DetectorFactory
     from dxtbx.model.goniometer import Goniometer, GoniometerFactory
     from dxtbx.model import Scan, ScanFactory
@@ -119,7 +119,7 @@ def __init__(self, **kwargs):
     self._detector_instance = detector_instance
 
     beam_instance = self._beam()
-    assert(isinstance(beam_instance, Beam))
+    assert(isinstance(beam_instance, BeamBase))
     self._beam_instance = beam_instance
 
     scan_instance = self._scan()