diff --git a/pandas/core/base.py b/pandas/core/base.py
index f648af85b68c5..1ad5acd9f136c 100644
--- a/pandas/core/base.py
+++ b/pandas/core/base.py
@@ -440,7 +440,10 @@ def nunique(self, dropna=True):
         -------
         nunique : int
         """
-        return len(self.value_counts(dropna=dropna))
+        result = self.unique()
+        if dropna:
+            return len(result) - np.isnan(np.sum(result))
+        return len(result)
 
     def factorize(self, sort=False, na_sentinel=-1):
         """