I am trying to implement the contract below in REMIX.

If I try to pass parameters to appendTags as "tag1", I get an error as Error:

invalid bytes32 value (arg="", type="string", value="tag1")

How can I pass parameters to the function appendTags in REMIX ?

contract Post{

struct tags{
bytes32[] tagged; 

mapping(address => tags) adminTags;

function appendTags(address addrs, bytes32 tag) public { 

function getTagsofAdmin(address addrs) public view returns(bytes32[]){ 
return adminTags[addrs].tagged; 

  • “Tag” is a string.
    – Rick Park
    Oct 28, 2018 at 20:58
  • I did not understand well your problem. Use string as type for the variable.
    – Rick Park
    Oct 28, 2018 at 21:00

1 Answer 1


As the Solidity code specifies, you need to pass a bytes32 value as argument, not an ascii-encoded word. This is an hexadecimal-encoded value, so as a rule of thumb it should start with 0x.

Before passing your argument, you thus need to hex-encode it using for example one of web3 util's functions:

> web3.utils.fromAscii('tag1')

You can pass this hexadecimal value as an argument in the Remix console. Later when you call getTagsOfAdmin you'll simply need to apply the reverse transformation:

> web3.utils.toAscii('0x74616731')

Despite this added complexity, bytes32 prove a good alternative against strings because they have fixed length and hence require way less storage costs. Hope it helps!

PS: note that the above command-line examples use web3 v1.

  • Thanks for the explanation. @thomas-vanderstraeten
    – Girish
    Oct 28, 2018 at 21:07

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