BUG:Sanity check on merge parameters for correct exception pandas-dev#26824

Author: harshit-py

pandas/core/reshape/merge.py

@@ -1089,13 +1089,19 @@ def _validate_specification(self):
                     raise ValueError('len(left_on) must equal the number '
                                      'of levels in the index of "right"')
                 self.right_on = [None] * n
+            if not self.right_on:
+                raise ValueError('both left_on and right_on '
+                                 'should be passed')
         elif self.right_on is not None:
             n = len(self.right_on)
             if self.left_index:
                 if len(self.right_on) != self.left.index.nlevels:
                     raise ValueError('len(right_on) must equal the number '
                                      'of levels in the index of "left"')
                 self.left_on = [None] * n
+            if not self.left_on:
+                raise ValueError('both left_on and right_on '
+                                 'should be passed')
         if len(self.right_on) != len(self.left_on):
             raise ValueError("len(right_on) must equal len(left_on)")