diff --git a/imsim/ccd.py b/imsim/ccd.py
index f9189483..4ef57d0f 100644
--- a/imsim/ccd.py
+++ b/imsim/ccd.py
@@ -194,8 +194,8 @@ def parse(item, type, default):
         image.header['AMSTART'] = airmass
         image.header['AMEND'] = airmass  # wrong, does anyone care?
         image.header['FOCUSZ'] = parse('focusZ', float, 0.0)
-        image.header['ALTITUDE'] = parse('altitude', float, None)
-        image.header['AZIMUTH'] = parse('azimuth', float, None)
+        image.header['ALTITUDE'] = parse('altitude', float, 'N/A')
+        image.header['AZIMUTH'] = parse('azimuth', float, 'N/A')
 
         # If there's anything left in header_vals, add it to the header.
         for k in header_vals: