diff --git a/iris/sg/_spectrograph.py b/iris/sg/_spectrograph.py
index 4a2a284..65aa846 100644
--- a/iris/sg/_spectrograph.py
+++ b/iris/sg/_spectrograph.py
@@ -255,6 +255,10 @@ def from_fits(
             raise ValueError(f"{window=} not in {windows=}")
 
         hdu_prototype = hdul_prototype[index_window]
+
+        if hdu_prototype.header["CDELT3"] == 0:
+            hdu_prototype.header["CDELT3"] = 1
+
         wcs_prototype = astropy.wcs.WCS(hdu_prototype)
 
         axes_wcs = list(reversed(wcs_prototype.axis_type_names))
@@ -293,6 +297,9 @@ def from_fits(
             hdu = hdul[index_window]
             hdu_aux = hdul[~1]
 
+            if hdu.header["CDELT3"] == 0:
+                hdu.header["CDELT3"] = 1
+
             detector_type = hdul[0].header[f"TDET{index_window}"]
 
             key_timedelta = "EXPTIMEF" if "FUV" in detector_type else "EXPTIMEN"
diff --git a/iris/sg/_spectrograph_test.py b/iris/sg/_spectrograph_test.py
index c1702b7..76d4cb2 100644
--- a/iris/sg/_spectrograph_test.py
+++ b/iris/sg/_spectrograph_test.py
@@ -12,7 +12,11 @@
             time_start=astropy.time.Time("2021-09-23T06:00"),
             time_stop=astropy.time.Time("2021-09-23T07:00"),
             nrt=True,
-        )
+        ),
+        iris.sg.SpectrographObservation.from_time_range(
+            time_start=astropy.time.Time("2019-01-23T02:49:03"),
+            time_stop=astropy.time.Time("2019-01-23T02:50:03"),
+        ),
     ],
 )
 class TestSpectrographObservation: