diff --git a/src/library/vm/vm.h b/src/library/vm/vm.h
index 556ff44a23..4a99b98ed9 100644
--- a/src/library/vm/vm.h
+++ b/src/library/vm/vm.h
@@ -236,7 +236,7 @@ public:
 
     unsigned get_nargs() const {
         lean_assert(m_op == opcode::InvokeGlobal || m_op == opcode::Closure);
-        return m_fn_idx;
+        return m_nargs;
     }
 
     unsigned get_idx() const {