golang graphql variable types

in my golang project I use https://github.com/shurcooL/graphql for graphql queries and I need to make the following mutation:

mutation createPromotion($input: CreatePromotionInput) {
  createPromotion(input: $input)
}

with the following graphql variables:

{
 "input": {
    "channel": "my_channel",
    "offerType": "my_offer_type",
   }
}

In my code it looks like this:

type CreatePromotion struct {
    CreatePromotion graphql.String
}

func (g *Gate) CreatePromotion(ctx context.Context) (*CreatePromotion, error) {
var mutation struct {
    CreatePromotion CreatePromotion `graphql:"createPromotion(input: {channel: $channel, 
  offerType: $offerType})"`
}

variables := map[string]interface{}{
    "channel":    graphql.String("my_channel"),
    "offerType":  graphql.String("my_offer_type"),//error is here
}
err := g.graphqlClient.Mutate(ctx, &mutation, variables)
if err != nil {
    return nil, err
 }
 return &mutation.CreatePromotion, nil
}

But when I execute my query I get the following error: Variable "$offerType" of type "String!" used in position expecting type "OfferType!". Unfortunately I neither have idea what type OfferType! is, nor how am I expected to make it.

Any ideas how to fix it would be welcome. Thank you.