diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7e482cce0..24cd6926f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -48,8 +48,10 @@ release.
   - ISIS-SPICE and NAIF-SPICE drivers: subtract 0.5 from the detector center (ISIS 0.5-based to CSM 0-based).
   - NAIF-SPICE driver: `focal_length` no longer raises `AttributeError` when `INS_FOCAL_LENGTH` is defined.
   - NAIF-SPICE driver: `ephemeris_start_time` uses the scalar from `utcToEt` (was a tuple). [#739](https://github.com/DOI-USGS/ale/pull/739)
+- `isd_generate` now fails with an error instead of writing an ISD with null required fields. [#754](https://github.com/DOI-USGS/ale/pull/754)
 - Fixed kernel quality check to allow quality keys from different missions [#758](https://github.com/DOI-USGS/ale/pull/758)
 
+
 ### Changed
 - `isd_generate`: default `--reduction` to `linear` (sampling every 10th line for line-scan images with >= 1000 lines). This avoids creating oversized ISDs (tens of thousands of pose samples) for long line-scan sensors such as LRO NAC, MRO CTX, and HRSC. Pass `--reduction none` to disable reduction and restore per-line sampling. [#744](https://github.com/DOI-USGS/ale/pull/744)
 
diff --git a/ale/base/__init__.py b/ale/base/__init__.py
index a03106b2f..fe4719a15 100644
--- a/ale/base/__init__.py
+++ b/ale/base/__init__.py
@@ -12,6 +12,14 @@ class WrongLabelTypeException(Exception):
     """
     pass
 
+class InvalidISDError(Exception):
+    """
+    Exception raised when a driver matches the label but the generated ISD is
+    invalid, for example a required field is null. Unlike a wrong-driver
+    exception, this is not retried against other drivers.
+    """
+    pass
+
 
 spiceql_mission_map = {
     "APOLLO_METRIC" : "apollo15_metric", 
diff --git a/ale/drivers/__init__.py b/ale/drivers/__init__.py
index b5cc32f68..8e440e5f8 100644
--- a/ale/drivers/__init__.py
+++ b/ale/drivers/__init__.py
@@ -11,7 +11,7 @@
 import os
 import os.path
 import traceback
-from ale.base import WrongInstrumentException, WrongLabelTypeException
+from ale.base import WrongInstrumentException, WrongLabelTypeException, InvalidISDError
 import logging
 
 from ale.formatters.formatter import to_isd
@@ -197,6 +197,7 @@ def load_from_label(label, props={}, formatter='ale', verbose=False, only_isis_s
 
     parsed_label = pre_parse_label(label)
 
+    invalid_isd_error = None
     for driver in drivers:
         if verbose:
             logger.info(f'Trying {driver.__name__}')
@@ -220,6 +221,13 @@ def load_from_label(label, props={}, formatter='ale', verbose=False, only_isis_s
         except WrongInstrumentException as e:
             if verbose:
                 logger.info(f"Wrong instrument id for driver {driver.__class__.__name__}: {e}. Skipping driver.")
+        except InvalidISDError as e:
+            # the driver produced an invalid ISD (a required field was null).
+            # Keep trying other drivers, but remember this so that if none
+            # succeed we surface this real error instead of a generic one.
+            invalid_isd_error = e
+            if verbose:
+                logger.info(f"Invalid ISD from driver {driver.__class__.__name__}: {e}. Skipping driver.")
         except Exception as e:
             if verbose:
                 traceback.print_exc()
@@ -227,6 +235,8 @@ def load_from_label(label, props={}, formatter='ale', verbose=False, only_isis_s
             logger.info(f'End {driver}\n')
     if verbose:
         logger.setLevel(logger_level)
+    if invalid_isd_error is not None:
+        raise invalid_isd_error
     raise Exception('No viable Driver for Label.')
 
 
diff --git a/ale/formatters/formatter.py b/ale/formatters/formatter.py
index 7c176aa34..00f0bfc4e 100644
--- a/ale/formatters/formatter.py
+++ b/ale/formatters/formatter.py
@@ -5,6 +5,7 @@
 import logging
 
 from ale.base.type_sensor import LineScanner, Framer, Radar, PushFrame
+from ale.base import InvalidISDError
 from ale import logger
 
 def to_isd(driver):
@@ -170,6 +171,14 @@ def to_isd(driver):
         isd['starting_detector_line'] = driver_data["detector_start_line"]
         isd['starting_detector_sample'] = driver_data["detector_start_sample"]
 
+        # these interior-orientation fields are required for any CSM model
+        io_required = ['focal_length', 'detector_center_line', 'detector_center_sample',
+                       'focal2pixel_lines', 'focal2pixel_samples']
+        io_null = [k for k in io_required if driver_data.get(k) is None]
+        if io_null:
+            raise InvalidISDError("Could not generate a valid ISD. Required field(s) "
+                                  "are null: " + ", ".join(io_null))
+
     j2000_rotation = frame_chain.compute_rotation(target_frame, J2000)
 
     instrument_position = {}