diff --git a/packages/backend-common/src/reading/BitbucketUrlReader.test.ts b/packages/backend-common/src/reading/BitbucketUrlReader.test.ts
index 5571a07898..974c84b2c2 100644
--- a/packages/backend-common/src/reading/BitbucketUrlReader.test.ts
+++ b/packages/backend-common/src/reading/BitbucketUrlReader.test.ts
@@ -242,5 +242,17 @@ describe('BitbucketUrlReader', () => {
 
       expect(response.etag).toBe('12ab34cd56ef');
     });
+
+    it('should throw error when apiBaseUrl is missing', () => {
+      expect(() => {
+        /* eslint-disable no-new */
+        new BitbucketUrlReader(
+          {
+            host: 'bitbucket.mycompany.net',
+          },
+          { treeResponseFactory },
+        );
+      }).toThrowError('must configure an explicit apiBaseUrl');
+    });
   });
 });
diff --git a/packages/backend-common/src/reading/GithubUrlReader.test.ts b/packages/backend-common/src/reading/GithubUrlReader.test.ts
index 975be85ccc..fae3f3ba12 100644
--- a/packages/backend-common/src/reading/GithubUrlReader.test.ts
+++ b/packages/backend-common/src/reading/GithubUrlReader.test.ts
@@ -376,5 +376,20 @@ describe('GithubUrlReader', () => {
       };
       await expect(fnGithub).rejects.toThrow(NotFoundError);
     });
+
+    it('should throw error when apiBaseUrl is missing', () => {
+      expect(() => {
+        /* eslint-disable no-new */
+        new GithubUrlReader(
+          {
+            host: 'ghe.mycompany.net',
+          },
+          {
+            treeResponseFactory,
+            credentialsProvider: mockCredentialsProvider,
+          },
+        );
+      }).toThrowError('must configure an explicit apiBaseUrl');
+    });
   });
 });
diff --git a/packages/backend-common/src/reading/GitlabUrlReader.test.ts b/packages/backend-common/src/reading/GitlabUrlReader.test.ts
index 90acfd1ab9..b18e7a4294 100644
--- a/packages/backend-common/src/reading/GitlabUrlReader.test.ts
+++ b/packages/backend-common/src/reading/GitlabUrlReader.test.ts
@@ -379,5 +379,17 @@ describe('GitlabUrlReader', () => {
       };
       await expect(fnGithub).rejects.toThrow(NotFoundError);
     });
+
+    it('should throw error when apiBaseUrl is missing', () => {
+      expect(() => {
+        /* eslint-disable no-new */
+        new GitlabUrlReader(
+          {
+            host: 'gitlab.mycompany.com',
+          },
+          { treeResponseFactory },
+        );
+      }).toThrowError('must configure an explicit apiBaseUrl');
+    });
   });
 });
diff --git a/packages/backend-common/src/reading/GitlabUrlReader.ts b/packages/backend-common/src/reading/GitlabUrlReader.ts
index 22316f6895..8a763d1259 100644
--- a/packages/backend-common/src/reading/GitlabUrlReader.ts
+++ b/packages/backend-common/src/reading/GitlabUrlReader.ts
@@ -51,6 +51,12 @@ export class GitlabUrlReader implements UrlReader {
     deps: { treeResponseFactory: ReadTreeResponseFactory },
   ) {
     this.treeResponseFactory = deps.treeResponseFactory;
+
+    if (!config.apiBaseUrl) {
+      throw new Error(
+        `GitLab integration for '${config.host}' must configure an explicit apiBaseUrl`,
+      );
+    }
   }
 
   async read(url: string): Promise<Buffer> {