diff --git a/src/vm.cpp b/src/vm.cpp
index 546d8d0..feb9a73 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -292,6 +292,7 @@ namespace wren {
 	}
 
 	void VM::variable(const char* module, const char* name, int slot) {
+		assert(slot_count() >= 1);
 		wrenGetVariable(m_local->wvm, module, name, slot);
 	}