Pandas ValueError: You are trying to merge on object and int64 columns

Updated: February 21, 2024 By: Guest Contributor Post a comment

The Error

Encountering a ValueError when merging dataframes in Pandas is a common issue, especially when the merging columns in the two dataframes are of different data types. This error typically reads Pandas ValueError: You are trying to merge on object and int64 columns. Understanding the root cause and knowing how to address it is key to smooth data manipulation.

Reasons for Error

The error arises due to a mismatch in data types of the column(s) being used to merge two pandas dataframes. Pandas requires that the dtype of the key columns being merged on is the same. If one column is an integer (int64) and the other is a string/object, the operation fails.

Solution 1: Convert Column Data Types

Ensuring both columns have the same data type is the most straightforward way to solve the problem. You can convert the data type of one column to match the other.

Steps to follow:

  1. Identify the column data types in both dataframes.
  2. Use the astype() method to convert the datatype of one column to match the other.
  3. Proceed with the merge operation.

Code example:

df_merged = pd.merge(df1, df2, on='key_column')

Notes: This method is straightforward and effective. However, it’s important to ensure that the conversion does not lead to data loss. For example, converting an identifier that begins with zeroes from a string to an integer would remove those leading zeroes.

Solution 2: Using Pandasmerge() Method with validate Option

An alternative solution is to use the validate option in Pandas’ merge() method, which doesn’t require changing data types but checks whether the merge keys from both frames are properly aligned and unique.


  1. Ensure that your key columns are correctly named and identified.
  2. Use pd.merge() and include the validate argument.
  3. Adjust the validation rule based on your merge operation (e.g., ‘one_to_one’, ‘one_to_many’).

Code example:

df_merged = pd.merge(df1, df2, on='key_column', validate='one_to_one')

Notes: While this solution does not solve the datatype mismatch issue directly, it’s useful for ensuring that your merge operation makes logical sense and might prevent other errors. It requires having a good understanding of the nature of the data and the relationship between the dataframes to be merged.

Solution 3: Ensuring Compatible Data Types Beforehand

If you frequently merge dataframes, it’s a good practice to ensure that key columns have compatible data types from the moment they are created or imported. This can save time and prevent errors down the line.

Steps to implement:

  • When loading data, use the dtype argument in read_csv(), read_sql(), or other pandas data import functions to specify correct data types.
  • Maintain consistency in data types across your data processing pipeline.


df = pd.read_csv('data.csv', dtype={'key_column': 'int64'})

Notes: This preemptive approach can significantly enhance data handling efficiency. However, it requires upfront knowledge about your data, which might not always be available.


Dealing with the Pandas ValueError: You are trying to merge on object and int64 columns can be frustrating, but understanding the solutions outlined here can help you resolve the issue. Each solution has its own benefits and limitations, and the best approach depends on your specific data and requirements.