diff --git a/composer.json b/composer.json
index e1ff8e96..6f7b6663 100644
--- a/composer.json
+++ b/composer.json
@@ -11,7 +11,8 @@
         "symfony/expression-language": "^4.4",
         "symfony/finder": "^4.4",
         "symfony/yaml": "^4.4",
-        "twig/twig": "^2.13"
+        "twig/twig": "^2.13",
+        "ext-json": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^9.5",
diff --git a/src/Types/Harness/Repository/Repository.php b/src/Types/Harness/Repository/Repository.php
index 668263d8..91756564 100644
--- a/src/Types/Harness/Repository/Repository.php
+++ b/src/Types/Harness/Repository/Repository.php
@@ -75,7 +75,13 @@ private function importPackagesFromSources()
                 continue;
             }
 
-            $this->packages = array_merge($this->packages, json_decode(file_get_contents($source['url']), true));
+            $packages = json_decode(file_get_contents($source['url']), true, JSON_THROW_ON_ERROR);
+
+            if ($packages === null) {
+                throw new Exception(sprintf('Response of %s can\'t be decoded as json.', $source['url']));
+            }
+
+            $this->packages = array_merge($this->packages, $packages);
             $this->sources[$k]['imported'] = true;
         }
     }