diff --git a/src/lib-sieve/plugins/vnd.dovecot/environment/ext-vnd-environment-common.h b/src/lib-sieve/plugins/vnd.dovecot/environment/ext-vnd-environment-common.h
index 97b795e85ebfcb8ea941eeca4284f28e54dee8cb..16cf02aa520a41253f8a12a3cc59dd643a60451b 100644
--- a/src/lib-sieve/plugins/vnd.dovecot/environment/ext-vnd-environment-common.h
+++ b/src/lib-sieve/plugins/vnd.dovecot/environment/ext-vnd-environment-common.h
@@ -15,7 +15,7 @@ struct ext_vnd_environment_context {
 	const struct sieve_extension *var_ext;
 };
 
-const struct sieve_extension_def vnd_environment_extension;
+extern const struct sieve_extension_def vnd_environment_extension;
 
 /*
  * Operands