diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
index f1e1116a5a58..8c7a9149f804 100644
--- a/lib/matplotlib/axes/_base.py
+++ b/lib/matplotlib/axes/_base.py
@@ -2716,9 +2716,12 @@ def grid(self, b=None, which='major', axis='both', **kwargs):
         elif b is not None:
             b = _string_to_bool(b)
 
-        if axis == 'x' or axis == 'both':
+        if axis not in ['x', 'y', 'both']:
+            raise ValueError("The argument 'axis' must be one of 'x', 'y' or "
+                             "'both'.")
+        if axis in ['x', 'both']:
             self.xaxis.grid(b, which=which, **kwargs)
-        if axis == 'y' or axis == 'both':
+        if axis in ['y', 'both']:
             self.yaxis.grid(b, which=which, **kwargs)
 
     def ticklabel_format(self, *, axis='both', style='', scilimits=None,
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
index fdebfb2156ca..386185bb9efe 100644
--- a/lib/matplotlib/axis.py
+++ b/lib/matplotlib/axis.py
@@ -1438,9 +1438,17 @@ def grid(self, b=None, which='major', **kwargs):
 
         """
         if len(kwargs):
+            if not b and b is not None:  # something false-like but not None
+                warnings.warn('First parameter to grid() is false, but line '
+                              'properties are supplied. The grid will be '
+                              'enabled.')
             b = True
         which = which.lower()
+        if which not in ['major', 'minor', 'both']:
+            raise ValueError("The argument 'which' must be one of 'major', "
+                             "'minor' or 'both'.")
         gridkw = {'grid_' + item[0]: item[1] for item in kwargs.items()}
+
         if which in ['minor', 'both']:
             if b is None:
                 self._gridOnMinor = not self._gridOnMinor