diff --git a/protoc/flags.bzl b/protoc/flags.bzl
index 88bc01c..9f706a7 100644
--- a/protoc/flags.bzl
+++ b/protoc/flags.bzl
@@ -8,20 +8,24 @@ PROTOC_TOOLCHAIN_FLAGS = {
         That flag ALSO decouples how each built-in language rule (Java, Python, C++, etc.) locates the runtime.
         """,
     ),
-    "per_file_copt": struct(
-        default = "external/.*protobuf.*@--PROTOBUF_WAS_NOT_SUPPOSED_TO_BE_BUILT",
-        description = "Make sure protobuf is not built from source",
-    ),
-    "host_per_file_copt": struct(
-        default = "external/.*protobuf.*@--PROTOBUF_WAS_NOT_SUPPOSED_TO_BE_BUILT",
-        description = "Make sure protobuf is not built from source",
-    ),
-    "per_file_copt": struct(
-        default = "external/.*grpc.*@--GRPC_WAS_NOT_SUPPOSED_TO_BE_BUILT",
-        description = "Make sure grpc is not built from source",
-    ),
-    "host_per_file_copt": struct(
-        default = "external/.*grpc.*@--GRPC_WAS_NOT_SUPPOSED_TO_BE_BUILT",
-        description = "Make sure grpc is not built from source",
-    ),
+    "per_file_copt": [
+        struct(
+            default = "external/.*protobuf.*@--PROTOBUF_WAS_NOT_SUPPOSED_TO_BE_BUILT",
+            description = "Make sure protobuf is not built from source",
+        ),
+        struct(
+            default = "external/.*grpc.*@--GRPC_WAS_NOT_SUPPOSED_TO_BE_BUILT",
+            description = "Make sure grpc is not built from source",
+        ),
+    ],
+    "host_per_file_copt": [
+        struct(
+            default = "external/.*protobuf.*@--PROTOBUF_WAS_NOT_SUPPOSED_TO_BE_BUILT",
+            description = "Make sure protobuf is not built from source",
+        ),
+        struct(
+            default = "external/.*grpc.*@--GRPC_WAS_NOT_SUPPOSED_TO_BE_BUILT",
+            description = "Make sure grpc is not built from source",
+        ),
+    ],
 }