diff --git a/src/vm/wren_value.c b/src/vm/wren_value.c
index c49a3b6b..6a38193f 100644
--- a/src/vm/wren_value.c
+++ b/src/vm/wren_value.c
@@ -1089,6 +1089,9 @@ static void blackenFn(WrenVM* vm, ObjFn* fn)
   // Mark the constants.
   wrenGrayBuffer(vm, &fn->constants);
 
+  // Mark the module it belongs to, in case it's been unloaded.
+  wrenGrayObj(vm, fn->module);
+
   // Keep track of how much memory is still in use.
   vm->bytesAllocated += sizeof(ObjFn);
   vm->bytesAllocated += sizeof(uint8_t) * fn->code.capacity;