diff --git a/pkgs/build-support/dart/pub2nix/pubspec-lock.nix b/pkgs/build-support/dart/pub2nix/pubspec-lock.nix
index 456236e18e5e..73788f535a90 100644
--- a/pkgs/build-support/dart/pub2nix/pubspec-lock.nix
+++ b/pkgs/build-support/dart/pub2nix/pubspec-lock.nix
@@ -94,8 +94,9 @@ let
 
   mkPathDependencySource =
     name: details:
-    assert lib.assertMsg details.description.relative
-      "Only relative paths are supported - ${name} has an absolue path!";
+    assert
+      details.description.relative
+      || throw "Only relative paths are supported - ${name} has an absolue path!";
     (
       if lib.isDerivation src then
         src