diff --git a/modules/ixBidAdapter.js b/modules/ixBidAdapter.js
index 3384bb8f41b..f3ac8185f17 100644
--- a/modules/ixBidAdapter.js
+++ b/modules/ixBidAdapter.js
@@ -185,7 +185,8 @@ export const spec = {
 
       // If the bid request is for banner, then transform the bid request based on banner format.
       if (utils.deepAccess(validBidRequest, 'mediaTypes.banner') ||
-        validBidRequest.mediaType === 'banner') {
+        validBidRequest.mediaType === 'banner' ||
+        (validBidRequest.mediaType === undefined && utils.deepAccess(validBidRequest, 'mediaTypes.banner') === undefined)) {
         bannerImp = bidToBannerImp(validBidRequest);
         bannerImps.push(bannerImp);
       }
diff --git a/test/spec/modules/ixBidAdapter_spec.js b/test/spec/modules/ixBidAdapter_spec.js
index 0e00a64aab4..92a7190bcb6 100644
--- a/test/spec/modules/ixBidAdapter_spec.js
+++ b/test/spec/modules/ixBidAdapter_spec.js
@@ -272,6 +272,21 @@ describe('IndexexchangeAdapter', () => {
 
       expect(requestStringTimeout.data.t).to.be.undefined;
     });
+
+    it('should default to assuming media type is banner', () => {
+      const bidsWithoutMediaType = [
+        Object.assign({}, DEFAULT_BANNER_VALID_BID[0])
+      ];
+      delete bidsWithoutMediaType[0].mediaTypes;
+
+      const request = spec.buildRequests(bidsWithoutMediaType);
+      const payload = JSON.parse(request.data.r);
+
+      expect(payload.id).to.equal(bidsWithoutMediaType[0].bidderRequestId);
+      expect(payload.imp).to.exist;
+      expect(payload.imp).to.be.an('array');
+      expect(payload.imp).to.have.lengthOf(1);
+    });
   });
 
   describe('interpretResponseBanner', () => {