Fix error when a PrimaryHDU has a "GROUPS = 1" keyword

astropy/io/fits/hdu/base.py

@@ -855,16 +855,7 @@
         """
         # The SIMPLE keyword must be in the first card
         card = header.cards[0]
-
-        # The check that 'GROUPS' is missing is a bit redundant, since the
-        # match_header for GroupsHDU will always be called before this one.
-        if card.keyword == "SIMPLE":
-            if "GROUPS" not in header and card.value is False:
-                return True
-            else:
-                raise InvalidHDUException
-        else:
-            return False
+        return card.keyword == "SIMPLE" and card.value is False
 
     @property
     def size(self):

astropy/io/fits/hdu/image.py

@@ -1142,7 +1142,7 @@ def match_header(cls, header):
         # keyword to be True/False, have to check the value
         return (
             card.keyword == "SIMPLE"
-            and ("GROUPS" not in header or header["GROUPS"] != True)  # noqa: E712
+            and ("GROUPS" not in header or header["GROUPS"] is not True)
             and card.value
         )
 

astropy/io/fits/tests/test_groups.py

@@ -243,3 +243,11 @@ def test_group_bad_naxis(self):
             assert len(hdul) == 1
             assert hdul[0].header["GROUPS"]
             assert hdul[0].data is None
+
+    def test_not_groups_file(self):
+        hdu = fits.PrimaryHDU()
+        hdu.header["GROUPS"] = (1, "not a groups HDU")
+        hdu.writeto(self.temp("not_groups.fits"))
+        with fits.open(self.temp("not_groups.fits")) as hdul:
+            assert hdul[0].header["GROUPS"] == 1
+            assert hdul[0].header.comments["GROUPS"] == "not a groups HDU"

docs/changes/io.fits/14998.bugfix.rst

@@ -0,0 +1,2 @@
+Fix crash when a PrimaryHDU has a GROUPS keyword with a non-boolean value (i.e.
+not a random-groups HDU).