diff --git a/xarray/backends/common.py b/xarray/backends/common.py
index fa3ee19f542..63c4c956f86 100644
--- a/xarray/backends/common.py
+++ b/xarray/backends/common.py
@@ -241,7 +241,7 @@ def encode_attribute(self, a):
         """encode one attribute"""
         return a
 
-    def set_dimension(self, d, l):  # pragma: no cover
+    def set_dimension(self, dim, length):  # pragma: no cover
         raise NotImplementedError()
 
     def set_attribute(self, k, v):  # pragma: no cover
diff --git a/xarray/backends/memory.py b/xarray/backends/memory.py
index bee6521bce2..17095d09651 100644
--- a/xarray/backends/memory.py
+++ b/xarray/backends/memory.py
@@ -40,6 +40,6 @@ def set_attribute(self, k, v):
         # copy to imitate writing to disk.
         self._attributes[k] = copy.deepcopy(v)
 
-    def set_dimension(self, d, l, unlimited_dims=None):
+    def set_dimension(self, dim, length, unlimited_dims=None):
         # in this model, dimensions are accounted for in the variables
         pass