diff --git a/tests/webidl/output_ALL.txt b/tests/webidl/output_ALL.txt
index 5a7788263c22b..c50dd16489819 100644
--- a/tests/webidl/output_ALL.txt
+++ b/tests/webidl/output_ALL.txt
@@ -63,6 +63,7 @@ object
 21.12
 198
 getAsArray: 24
+Inner::+= => 2
 0
 1
 34,34
diff --git a/tests/webidl/output_DEFAULT.txt b/tests/webidl/output_DEFAULT.txt
index afc0bb3b2a609..d0d3faf3898b0 100644
--- a/tests/webidl/output_DEFAULT.txt
+++ b/tests/webidl/output_DEFAULT.txt
@@ -63,6 +63,7 @@ object
 21.12
 198
 getAsArray: 24
+Inner::+= => 2
 0
 1
 34,34
diff --git a/tests/webidl/output_FAST.txt b/tests/webidl/output_FAST.txt
index afc0bb3b2a609..d0d3faf3898b0 100644
--- a/tests/webidl/output_FAST.txt
+++ b/tests/webidl/output_FAST.txt
@@ -63,6 +63,7 @@ object
 21.12
 198
 getAsArray: 24
+Inner::+= => 2
 0
 1
 34,34
diff --git a/tests/webidl/post.js b/tests/webidl/post.js
index abff14182fcd1..445a14511b3c6 100644
--- a/tests/webidl/post.js
+++ b/tests/webidl/post.js
@@ -124,6 +124,7 @@ bv2.getAnother().PrintFloat(21.12);
 console.log(new TheModule.Inner().get());
 console.log('getAsArray: ' + new TheModule.Inner().getAsArray(12));
 new TheModule.Inner().mul(2);
+new TheModule.Inner().incInPlace(new TheModule.Inner());
 
 console.log(TheModule.enum_value1);
 console.log(TheModule.enum_value2);
diff --git a/tests/webidl/test.h b/tests/webidl/test.h
index 016a3df5432fe..c228e34634e5e 100644
--- a/tests/webidl/test.h
+++ b/tests/webidl/test.h
@@ -82,10 +82,15 @@ struct VoidPointerUser {
 
 namespace Space {
   struct Inner {
-    Inner() {}
+    int value;
+    Inner() : value(1) {}
     int get() { return 198; }
     Inner& operator*=(float x) { return *this; }
     int operator[](int x) { return x*2; }
+    void operator+=(const Inner& other) {
+      value += other.value;
+      printf("Inner::+= => %d\n", value);
+    }
   };
 }
 
diff --git a/tests/webidl/test.idl b/tests/webidl/test.idl
index 77c511dc7e976..98b9fcfc6b379 100644
--- a/tests/webidl/test.idl
+++ b/tests/webidl/test.idl
@@ -77,6 +77,7 @@ interface Inner {
   long get();
   [Operator="*=", Ref] Inner mul(float x);
   [Operator="[]"] long getAsArray(long x);
+  [Operator="+="] void incInPlace([Const, Ref] Inner i);
 };
 
 enum AnEnum {
diff --git a/tools/webidl_binder.py b/tools/webidl_binder.py
index 136ccc06c4a31..3b16787ce1798 100644
--- a/tools/webidl_binder.py
+++ b/tools/webidl_binder.py
@@ -504,7 +504,7 @@ def render_function(class_name, func_name, sigs, return_type, non_pointer, copy,
       if class_name != func_scope:
         # this function comes from an ancestor class; for operators, we must cast it
         cast_self = 'dynamic_cast<' + type_to_c(func_scope) + '>(' + cast_self + ')'
-      maybe_deref = '*' if sig[0] in interfaces else ''
+      maybe_deref = deref_if_nonpointer(raw[0])
       if '=' in operator:
         call = '(*%s %s %s%s)' % (cast_self, operator, maybe_deref, args[0])
       elif operator == '[]':