diff --git a/scala_proto/scala_proto.bzl b/scala_proto/scala_proto.bzl
index 8887ad301..d22e22c90 100644
--- a/scala_proto/scala_proto.bzl
+++ b/scala_proto/scala_proto.bzl
@@ -351,7 +351,7 @@ def _gen_proto_srcjar_impl(ctx):
         # Command line args to worker cannot be empty so using padding
         flags_arg = "-" + ",".join(ctx.attr.flags),
         # Command line args to worker cannot be empty so using padding
-        packages = "-" + ":".join(transitive_proto_paths)
+        packages = "-" + ":".join(depset(transitive = transitive_proto_paths).to_list())
     )
     argfile = ctx.actions.declare_file("%s_worker_input" % ctx.label.name, sibling = ctx.outputs.srcjar)
     ctx.actions.write(output=argfile, content=worker_content)
diff --git a/test_rules_scala.sh b/test_rules_scala.sh
index e84959294..3dec143b7 100755
--- a/test_rules_scala.sh
+++ b/test_rules_scala.sh
@@ -805,3 +805,4 @@ $runner test_scala_import_library_passes_labels_of_direct_deps
 $runner java_toolchain_javacopts_are_used
 $runner bazel run test/src/main/scala/scala/test/classpath_resources:classpath_resource
 $runner test_scala_classpath_resources_expect_warning_on_namespace_conflict
+$runner bazel build //test_expect_failure/proto_source_root/... --strict_proto_deps=off
\ No newline at end of file