diff --git a/src/compiler/jsgen.js b/src/compiler/jsgen.js
index 640f8e88..3b52df4e 100644
--- a/src/compiler/jsgen.js
+++ b/src/compiler/jsgen.js
@@ -189,6 +189,7 @@ class VariableInput {
                 input = input._value;
             } else {
                 this.type = TYPE_UNKNOWN;
+                this._value = null;
                 return;
             }
         }