diff --git a/xpcom/idl-parser/xpidl/header.py b/xpcom/idl-parser/xpidl/header.py
index 15eba9dd7ef..467a60f5ef7 100644
--- a/xpcom/idl-parser/xpidl/header.py
+++ b/xpcom/idl-parser/xpidl/header.py
@@ -111,6 +111,19 @@ def paramlistAsNative(m, empty='void'):
                                            location=None,
                                            realtype=m.realtype)))
 
+    # Set any optional out params to default to nullptr. Skip if we just added
+    # extra non-optional args to l.
+    if len(l) == len(m.params):
+        paramIter = len(m.params) - 1
+        while (paramIter >= 0 and m.params[paramIter].optional and
+                m.params[paramIter].paramtype == "out"):
+            t = m.params[paramIter].type
+            # Strings can't be optional, so this shouldn't happen, but let's make sure:
+            if t == "AString" or t == "ACString" or t == "DOMString" or t == "AUTF8String":
+                break
+            l[paramIter] += " = nullptr"
+            paramIter -= 1
+
     if len(l) == 0:
         return empty