diff --git a/python/desc/imsim/camera_readout.py b/python/desc/imsim/camera_readout.py
index 531ba1a6..bcc5ccf7 100644
--- a/python/desc/imsim/camera_readout.py
+++ b/python/desc/imsim/camera_readout.py
@@ -461,6 +461,13 @@ def write_fits_file(self, outfile, overwrite=True, run_number=None,
         """
         output = fits.HDUList(fits.PrimaryHDU())
         output[0].header = self.eimage[0].header
+        # Since `.header` acts like an unordered dict when set like
+        # this, we need to re-insert the WCSAXES keyword to precede
+        # any other WCS keywords as stipulated in the FITS Standard,
+        # version 4.0, section 8.2.
+        wcsaxes = output[0].header['WCSAXES']
+        del output[0].header['WCSAXES']
+        output[0].header.insert(5, ('WCSAXES', wcsaxes, ''))
         if run_number is None:
             run_number = self.visit
         output[0].header['RUNNUM'] = str(run_number)