diff --git a/src/de/inetsoftware/jwebassembly/module/TypeManager.java b/src/de/inetsoftware/jwebassembly/module/TypeManager.java
index 7d78eff..d69e7ab 100644
--- a/src/de/inetsoftware/jwebassembly/module/TypeManager.java
+++ b/src/de/inetsoftware/jwebassembly/module/TypeManager.java
@@ -223,7 +223,7 @@ public class TypeManager {
      *             if any I/O error occur on loading or writing
      */
     void scanTypeHierarchy() throws IOException {
-        for( StructType type : structTypes.values() ) {
+        for( StructType type : new ArrayList<>( structTypes.values() ) ) {
             type.scanTypeHierarchy( options.functions, this, classFileLoader );
         }
     }