diff --git a/src/interpreter.cpp b/src/interpreter.cpp
index e0ac740..82a4f86 100644
--- a/src/interpreter.cpp
+++ b/src/interpreter.cpp
@@ -95,7 +95,8 @@ EvalResult eval(Node &ast, Memory &memory) {
                     eval(node.children[0], memory);
 
                     while (true) {
-                        int cond = bool_cast(eval(node.children[1], memory));
+                        EvalResult e1 = eval(node.children[1], memory);
+                        int cond = holds_alternative<monostate>(e1) || bool_cast(e1);
                         if (!cond) break;
 
                         try {