diff --git a/src/de/inetsoftware/jwebassembly/text/Function.java b/src/de/inetsoftware/jwebassembly/text/Function.java
index 33f4fc2..b88fc65 100644
--- a/src/de/inetsoftware/jwebassembly/text/Function.java
+++ b/src/de/inetsoftware/jwebassembly/text/Function.java
@@ -24,7 +24,7 @@ class Function {
 
     int                 id;
 
-    int                 typeId;
+    int                 typeId = -1; // -1 not init, use an invalid value to fail hard if it used without init
 
     final StringBuilder output = new StringBuilder();
 }