diff --git a/lang/types.h b/lang/types.h
index 2e7668b..3e316a0 100644
--- a/lang/types.h
+++ b/lang/types.h
@@ -5,9 +5,11 @@
 #define TYPES_H
 
 // Opaque type for an object
-typedef void *Object;
+struct object;
+typedef struct object *Object;
 
 // Opaque type for the VM state
-typedef void *VmState;
+struct vm_state;
+typedef struct vm_state *VmState;
 
 #endif