diff --git a/pandas/core/internals/api.py b/pandas/core/internals/api.py
index 3fbe324417c60..9d517fe55f808 100644
--- a/pandas/core/internals/api.py
+++ b/pandas/core/internals/api.py
@@ -10,6 +10,7 @@
 
 import numpy as np
 
+from pandas._libs.internals import BlockPlacement
 from pandas._typing import Dtype
 
 from pandas.core.dtypes.common import is_datetime64tz_dtype
@@ -58,4 +59,17 @@ def make_block(
         #  for e.g. pyarrow?
         values = DatetimeArray._simple_new(values, dtype=dtype)
 
+    if not isinstance(placement, BlockPlacement):
+        placement = BlockPlacement(placement)
+
+    if ndim is None:
+        # GH#38134 Block constructor now assumes ndim is not None
+        if not isinstance(values.dtype, np.dtype):
+            if len(placement) != 1:
+                ndim = 1
+            else:
+                ndim = 2
+        else:
+            ndim = values.ndim
+
     return klass(values, ndim=ndim, placement=placement)
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index b92ef3ec3b367..0ce746465cce4 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -170,6 +170,10 @@ def __init__(self, values, placement, ndim: int):
                 f"placement implies {len(self.mgr_locs)}"
             )
 
+        elif self.is_extension and self.ndim == 2 and len(self.mgr_locs) != 1:
+            # TODO(EA2D): check unnecessary with 2D EAs
+            raise AssertionError("block.size != values.size")
+
     @classmethod
     def _maybe_coerce_values(cls, values):
         """
@@ -185,7 +189,7 @@ def _maybe_coerce_values(cls, values):
         """
         return values
 
-    def _check_ndim(self, values, ndim):
+    def _check_ndim(self, values, ndim: int):
         """
         ndim inference and validation.
 
@@ -196,7 +200,7 @@ def _check_ndim(self, values, ndim):
         Parameters
         ----------
         values : array-like
-        ndim : int or None
+        ndim : int
 
         Returns
         -------
@@ -206,8 +210,7 @@ def _check_ndim(self, values, ndim):
         ------
         ValueError : the number of dimensions do not match
         """
-        if ndim is None:
-            ndim = values.ndim
+        assert isinstance(ndim, int)  # GH#38134 enforce this
 
         if self._validate_ndim:
             if values.ndim != ndim:
@@ -1481,33 +1484,6 @@ class ExtensionBlock(Block):
 
     values: ExtensionArray
 
-    def __init__(self, values, placement, ndim: int):
-        """
-        Initialize a non-consolidatable block.
-
-        'ndim' may be inferred from 'placement'.
-
-        This will call continue to call __init__ for the other base
-        classes mixed in with this Mixin.
-        """
-
-        # Placement must be converted to BlockPlacement so that we can check
-        # its length
-        if not isinstance(placement, libinternals.BlockPlacement):
-            placement = libinternals.BlockPlacement(placement)
-
-        # Maybe infer ndim from placement
-        if ndim is None:
-            if len(placement) != 1:
-                ndim = 1
-            else:
-                ndim = 2
-        super().__init__(values, placement, ndim=ndim)
-
-        if self.ndim == 2 and len(self.mgr_locs) != 1:
-            # TODO(EA2D): check unnecessary with 2D EAs
-            raise AssertionError("block.size != values.size")
-
     @property
     def shape(self) -> Shape:
         # TODO(EA2D): override unnecessary with 2D EAs