diff --git a/image/docker/schema1.py b/image/docker/schema1.py
index f154b7d21..14e77ad83 100644
--- a/image/docker/schema1.py
+++ b/image/docker/schema1.py
@@ -25,8 +25,8 @@ logger = logging.getLogger(__name__)
 # Content Types
 DOCKER_SCHEMA1_MANIFEST_CONTENT_TYPE = 'application/vnd.docker.distribution.manifest.v1+json'
 DOCKER_SCHEMA1_SIGNED_MANIFEST_CONTENT_TYPE = 'application/vnd.docker.distribution.manifest.v1+prettyjws'
-DOCKER_SCHEMA1_CONTENT_TYPES = [DOCKER_SCHEMA1_MANIFEST_CONTENT_TYPE,
-                                DOCKER_SCHEMA1_SIGNED_MANIFEST_CONTENT_TYPE]
+DOCKER_SCHEMA1_CONTENT_TYPES = {DOCKER_SCHEMA1_MANIFEST_CONTENT_TYPE,
+                                DOCKER_SCHEMA1_SIGNED_MANIFEST_CONTENT_TYPE}
 
 # Keys for signature-related data
 DOCKER_SCHEMA1_SIGNATURES_KEY = 'signatures'
diff --git a/image/docker/schema2.py b/image/docker/schema2.py
index 4a69ab8bb..504f5df80 100644
--- a/image/docker/schema2.py
+++ b/image/docker/schema2.py
@@ -7,5 +7,5 @@ https://github.com/docker/distribution/blob/master/docs/spec/manifest-v2-2.md
 # Content Types
 DOCKER_SCHEMA2_MANIFEST_CONTENT_TYPE = 'application/vnd.docker.distribution.manifest.v2+json'
 DOCKER_SCHEMA2_MANIFESTLIST_CONTENT_TYPE = 'application/vnd.docker.distribution.manifest.list.v2+json'
-DOCKER_SCHEMA2_CONTENT_TYPES = [DOCKER_SCHEMA2_MANIFEST_CONTENT_TYPE,
-                                DOCKER_SCHEMA2_MANIFESTLIST_CONTENT_TYPE]
+DOCKER_SCHEMA2_CONTENT_TYPES = {DOCKER_SCHEMA2_MANIFEST_CONTENT_TYPE,
+                                DOCKER_SCHEMA2_MANIFESTLIST_CONTENT_TYPE}