jsonapi.services.yml                 |  2 +-
 src/StackMiddleware/FormatSetter.php | 14 ++++++++------
 2 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/jsonapi.services.yml b/jsonapi.services.yml
index c209631..9addb16 100644
--- a/jsonapi.services.yml
+++ b/jsonapi.services.yml
@@ -128,7 +128,7 @@ services:
   jsonapi.http_middleware.format_setter:
     class: Drupal\jsonapi\StackMiddleware\FormatSetter
     calls:
-      - [setJsonApiPathPrefix, ['@jsonapi.resource_type.repository']]
+      - [setJsonApiBasePath, ['@jsonapi.resource_type.repository']]
     tags:
       # Set priority to 201 so it happens right before the page cache
       # middleware (priority 200)has the opportunity to respond.
diff --git a/src/StackMiddleware/FormatSetter.php b/src/StackMiddleware/FormatSetter.php
index 4072b8d..7befaba 100644
--- a/src/StackMiddleware/FormatSetter.php
+++ b/src/StackMiddleware/FormatSetter.php
@@ -22,11 +22,11 @@ class FormatSetter implements HttpKernelInterface {
   protected $httpKernel;
 
   /**
-   * The JSON API path prefix.
+   * The JSON API base path.
    *
    * @var string
    */
-  protected $jsonApiPathPrefix;
+  protected $jsonApiBasePath;
 
   /**
    * Constructs a FormatSetter object.
@@ -39,13 +39,15 @@ class FormatSetter implements HttpKernelInterface {
   }
 
   /**
-   * Sets the JSON API path prefix.
+   * Sets the JSON API base path.
    *
    * @param \Drupal\jsonapi\ResourceType\ResourceTypeRepositoryInterface $resource_type_repository
    *   The JSON API resource type repository.
    */
-  public function setJsonApiPathPrefix(ResourceTypeRepositoryInterface $resource_type_repository) {
-    $this->jsonApiPathPrefix = $resource_type_repository->getPathPrefix();
+  public function setJsonApiBasePath(ResourceTypeRepositoryInterface $resource_type_repository) {
+    $this->jsonApiBasePath = $resource_type_repository->getBasePath();
+    assert($this->jsonApiBasePath[0] === '/');
+    assert(substr($this->jsonApiBasePath, -1) !== '/');
   }
 
   /**
@@ -79,7 +81,7 @@ class FormatSetter implements HttpKernelInterface {
     // Check if the path indicates that the request intended to target a JSON
     // API route (but may not have because of an incorrect parameter or minor
     // typo).
-    $jsonapi_route_intended = strpos($request->getPathInfo(), "/{$this->jsonApiPathPrefix}/") !== FALSE;
+    $jsonapi_route_intended = strpos($request->getPathInfo(), "{$this->jsonApiBasePath}/") !== FALSE;
     // Check if the 'Accept' header includes the JSON API MIME type.
     $request_has_jsonapi_media_type = count(array_filter($request->getAcceptableContentTypes(), function ($accept) {
       return strpos($accept, 'application/vnd.api+json') === 0;