diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index 4d83d1d78ee35..7ca4a1165c782 100644
--- a/pandas/core/reshape/merge.py
+++ b/pandas/core/reshape/merge.py
@@ -916,11 +916,13 @@ def _maybe_coerce_merge_keys(self):
             # Houston, we have a problem!
             # let's coerce to object
             if name in self.left.columns:
-                typ = lk.categories.dtype if is_categorical_dtype(lk) else object
+                cat = is_categorical_dtype(lk)
+                typ = lk.categories.dtype if cat else object
                 self.left = self.left.assign(
                     **{name: self.left[name].astype(typ)})
             if name in self.right.columns:
-                typ = rk.categories.dtype if is_categorical_dtype(rk) else object
+                cat = is_categorical_dtype(rk)
+                typ = rk.categories.dtype if cat else object
                 self.right = self.right.assign(
                     **{name: self.right[name].astype(typ)})