diff --git a/packages/core/src/__tests__/httpClient/basicHttpClient.unit.spec.ts b/packages/core/src/__tests__/httpClient/basicHttpClient.unit.spec.ts
new file mode 100644
index 0000000000..d98f24632e
--- /dev/null
+++ b/packages/core/src/__tests__/httpClient/basicHttpClient.unit.spec.ts
@@ -0,0 +1,58 @@
+// Copyright 2020 Cognite AS
+
+// @ts-ignore
+import {
+  headersWithDefaultField,
+  HttpHeaders,
+} from '../../httpClient/basicHttpClient';
+
+function lengthOfHttpHeaders(headers?: HttpHeaders): number {
+  let counter = 0;
+  for (const _ in headers) {
+    counter += 1;
+  }
+  return counter;
+}
+
+describe('basicHttpClient', () => {
+  describe('headersWithDefaultField', () => {
+    test('add missing', () => {
+      const emptyHeaders: HttpHeaders = {};
+      const alteredEmptyHeaders = headersWithDefaultField(
+        emptyHeaders,
+        'Accept',
+        'application/json'
+      );
+      expect(lengthOfHttpHeaders(alteredEmptyHeaders)).toEqual(1);
+      expect('Accept' in alteredEmptyHeaders).toBeTruthy();
+    });
+    test('not overwrite existing', () => {
+      const mediaType = 'image/png';
+      const emptyHeaders: HttpHeaders = {
+        Accept: mediaType,
+      };
+      const alteredEmptyHeaders = headersWithDefaultField(
+        emptyHeaders,
+        'Accept',
+        'application/json'
+      );
+      expect(lengthOfHttpHeaders(alteredEmptyHeaders)).toEqual(1);
+      expect('Accept' in alteredEmptyHeaders).toBeTruthy();
+      expect(alteredEmptyHeaders['Accept']).toEqual(mediaType);
+    });
+    test('to be case insensitive', () => {
+      const mediaType = 'image/png';
+      const emptyHeaders: HttpHeaders = {
+        accept: mediaType,
+      };
+      const alteredEmptyHeaders = headersWithDefaultField(
+        emptyHeaders,
+        'Accept',
+        'application/json'
+      );
+      expect(lengthOfHttpHeaders(alteredEmptyHeaders)).toEqual(1);
+      expect('accept' in alteredEmptyHeaders).toBeTruthy();
+      expect(alteredEmptyHeaders['accept']).toEqual(mediaType);
+    });
+  });
+});
diff --git a/packages/core/src/httpClient/basicHttpClient.ts b/packages/core/src/httpClient/basicHttpClient.ts
index cf60a17d1d..3e2f16c4f6 100644
--- a/packages/core/src/httpClient/basicHttpClient.ts
+++ b/packages/core/src/httpClient/basicHttpClient.ts
@@ -179,10 +179,11 @@ export class BasicHttpClient {
     request: HttpRequest
   ): Promise<HttpResponse<ResponseType>> {
     const url = this.constructUrl(request.path, request.params);
-    const headers: HttpHeaders = {
-      Accept: 'application/json',
-      ...request.headers,
-    };
+    const headers = headersWithDefaultField(
+      request.headers,
+      'Accept',
+      'application/json'
+    );
     let body = request.data;
     if (isJson(body)) {
       body = BasicHttpClient.transformRequestBody(body);
@@ -224,6 +225,27 @@ export class BasicHttpClient {
   }
 }
 
+function lowercaseHeadersKeys(headers: HttpHeaders): string[] {
+  const keys: string[] = [];
+  for (const key in headers) {
+    keys.push(key.toLowerCase());
+  }
+  return keys;
+}
+
+export function headersWithDefaultField(
+  headers: HttpHeaders = {},
+  fieldName: string,
+  fieldValue: string
+): HttpHeaders {
+  const lowercaseHeaders = lowercaseHeadersKeys(headers);
+  const lowercaseKey = fieldName.toLowerCase();
+  if (!lowercaseHeaders.includes(lowercaseKey)) {
+    headers[fieldName] = fieldValue;
+  }
+  return headers;
+}
+
 export interface HttpRequest extends HttpRequestOptions {
   path: string;
   method: HttpMethod;
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 39a1e13a8c..83c9f926be 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -2,6 +2,7 @@
 import * as Constants from './constants';
 import * as GraphUtils from './graphUtils';
 import * as TestUtils from './testUtils';
+
 export * from './types';
 
 export { MetadataMap } from './metadata';