diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
index c3d4aaf62d0b..dbe577174916 100644
--- a/lib/matplotlib/__init__.py
+++ b/lib/matplotlib/__init__.py
@@ -830,12 +830,18 @@ def rc_params_from_file(fname, fail_on_error=False, use_default_template=True):
     rcParamsOrig = RcParams(rcParams.copy())
     # This also checks that all rcParams are indeed listed in the template.
     # Assigning to rcsetup.defaultParams is left only for backcompat.
-    defaultParams = rcsetup.defaultParams = {
-        # We want to resolve deprecated rcParams, but not backend...
-        key: [(rcsetup._auto_backend_sentinel if key == "backend" else
-               rcParamsDefault[key]),
-              validator]
-        for key, validator in rcsetup._validators.items()}
+    defaultParams = {}
+    for key, validator in rcsetup._validators.items():
+        if key == 'backend':
+            defaultParams['backend'] = [rcsetup._auto_backend_sentinel,
+                                        validator]
+        elif key in rcParamsDefault.keys():
+            defaultParams[key] = [rcParamsDefault[key], validator]
+        else:
+            _api.warn_external(f'rcsetup key "{key}" not in the default'
+                                ' rcParams')
+    rcsetup.defaultParams = defaultParams
+
 if rcParams['axes.formatter.use_locale']:
     locale.setlocale(locale.LC_ALL, '')
 
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
index 4705b975d60a..a385edb3eace 100644
--- a/lib/matplotlib/tests/test_rcparams.py
+++ b/lib/matplotlib/tests/test_rcparams.py
@@ -13,6 +13,7 @@
 from matplotlib import _api, _c_internal_utils
 import matplotlib.pyplot as plt
 import matplotlib.colors as mcolors
+import matplotlib.rcsetup as rcsetup
 import numpy as np
 from matplotlib.rcsetup import (
     validate_bool,
@@ -31,6 +32,23 @@
     _listify_validator)
 
 
+def test_rc_validators_in_sync():
+    # make sure that matplotlibrc.template and rcsetup._validators
+    # are in sync.
+    rc = mpl._rc_params_in_file(
+            'matplotlibrc.template',
+            # Strip leading comment.
+            transform=lambda line: line[1:] if line.startswith("#") else line,
+            fail_on_error=True)
+    for key, validator in rcsetup._validators.items():
+        if (key not in mpl._deprecated_remain_as_none and
+                key not in mpl._deprecated_ignore_map and
+                key[0] != '_'):
+            assert key in list(rc.keys())
+    for key in list(rc.keys()):
+        assert key in list(rcsetup._validators.keys())
+
+
 def test_rcparams(tmpdir):
     mpl.rc('text', usetex=False)
     mpl.rc('lines', linewidth=22)