diff --git a/lib/datasource/packagist/index.js b/lib/datasource/packagist/index.js
index 6ee2be8783302d6cd563d46c2ff5b16212dceb3f..8adeb74eb952cf0250ac2cca759fd0b42df73ac4 100644
--- a/lib/datasource/packagist/index.js
+++ b/lib/datasource/packagist/index.js
@@ -255,15 +255,14 @@ async function getPkgReleases({ lookupName, registryUrls }) {
   logger.trace(`getPkgReleases(${lookupName})`);
 
   let res;
-  if (is.nonEmptyArray(registryUrls)) {
-    for (const regUrl of registryUrls) {
-      res = await packageLookup(regUrl, lookupName);
-      if (res) {
-        break;
-      }
+  const registries = is.nonEmptyArray(registryUrls)
+    ? registryUrls
+    : ['https://packagist.org'];
+  for (const regUrl of registries) {
+    res = await packageLookup(regUrl, lookupName);
+    if (res) {
+      break;
     }
-  } /* istanbul ignore next */ else {
-    logger.debug({ lookupName }, 'No registryUrls defined');
   }
   return res;
 }
diff --git a/test/datasource/__snapshots__/packagist.spec.js.snap b/test/datasource/__snapshots__/packagist.spec.js.snap
index 9ddefe86fbf5f7bdfdfeebbe00d22813dad7c75e..34885f5022cfa8515eca93b3d41fabc8a262d9ff 100644
--- a/test/datasource/__snapshots__/packagist.spec.js.snap
+++ b/test/datasource/__snapshots__/packagist.spec.js.snap
@@ -1,5 +1,90 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`datasource/packagist getPkgReleases adds packagist source implicitly 1`] = `
+Object {
+  "homepage": "https://github.com/drewm/mailchimp-api",
+  "name": "drewm/mailchimp-api",
+  "releases": Array [
+    Object {
+      "gitRef": "v1.0",
+      "releaseTimestamp": "2014-05-30T16:51:39+00:00",
+      "version": "1.0",
+    },
+    Object {
+      "gitRef": "v1.1",
+      "releaseTimestamp": "2015-07-07T15:38:25+00:00",
+      "version": "1.1",
+    },
+    Object {
+      "gitRef": "v2.0",
+      "releaseTimestamp": "2016-01-17T13:08:01+00:00",
+      "version": "2.0",
+    },
+    Object {
+      "gitRef": "v2.1",
+      "releaseTimestamp": "2016-01-30T16:12:54+00:00",
+      "version": "2.1",
+    },
+    Object {
+      "gitRef": "v2.1.1",
+      "releaseTimestamp": "2016-04-06T08:37:20+00:00",
+      "version": "2.1.1",
+    },
+    Object {
+      "gitRef": "v2.1.2",
+      "releaseTimestamp": "2016-04-06T12:41:37+00:00",
+      "version": "2.1.2",
+    },
+    Object {
+      "gitRef": "v2.1.3",
+      "releaseTimestamp": "2016-04-12T09:09:47+00:00",
+      "version": "2.1.3",
+    },
+    Object {
+      "gitRef": "v2.2",
+      "releaseTimestamp": "2016-04-23T12:43:28+00:00",
+      "version": "2.2",
+    },
+    Object {
+      "gitRef": "v2.2.1",
+      "releaseTimestamp": "2016-04-23T18:00:21+00:00",
+      "version": "2.2.1",
+    },
+    Object {
+      "gitRef": "v2.2.2",
+      "releaseTimestamp": "2016-07-01T09:58:24+00:00",
+      "version": "2.2.2",
+    },
+    Object {
+      "gitRef": "v2.2.3",
+      "releaseTimestamp": "2016-07-01T15:53:33+00:00",
+      "version": "2.2.3",
+    },
+    Object {
+      "gitRef": "v2.2.4",
+      "releaseTimestamp": "2016-07-01T15:53:33+00:00",
+      "version": "2.2.4",
+    },
+    Object {
+      "gitRef": "v2.3",
+      "releaseTimestamp": "2016-12-21T14:50:24+00:00",
+      "version": "2.3",
+    },
+    Object {
+      "gitRef": "v2.4",
+      "releaseTimestamp": "2017-02-16T13:24:20+00:00",
+      "version": "2.4",
+    },
+    Object {
+      "gitRef": "v2.5",
+      "releaseTimestamp": "2018-02-16T15:31:05+00:00",
+      "version": "2.5",
+    },
+  ],
+  "sourceUrl": "https://github.com/drewm/mailchimp-api",
+}
+`;
+
 exports[`datasource/packagist getPkgReleases processes real versioned data 1`] = `
 Object {
   "homepage": "https://github.com/drewm/mailchimp-api",
diff --git a/test/datasource/packagist.spec.js b/test/datasource/packagist.spec.js
index 8046ab8a1a6abd1d0f1926d88441d9854813b4db..19d03c2a7ca5f5140f480406289f6bfff3ebc532 100644
--- a/test/datasource/packagist.spec.js
+++ b/test/datasource/packagist.spec.js
@@ -219,5 +219,16 @@ describe('datasource/packagist', () => {
         })
       ).toMatchSnapshot();
     });
+    it('adds packagist source implicitly', async () => {
+      got.mockReturnValueOnce({
+        body: JSON.parse(mailchimpJson),
+      });
+      expect(
+        await datasource.getPkgReleases({
+          ...config,
+          lookupName: 'drewm/mailchimp-api',
+        })
+      ).toMatchSnapshot();
+    });
   });
 });