diff --git a/lib/ast.js b/lib/ast.js
index 5f34dc5b..ae10abdf 100644
--- a/lib/ast.js
+++ b/lib/ast.js
@@ -425,8 +425,13 @@ var AST_ArrowParametersOrSeq = DEFNODE("ArrowParametersOrSeq", "expressions", {
                     default: default_seen_above,
                     names: ex.elements.map(to_fun_args)
                 });
-            } else if (ex instanceof AST_Assign) {
-                return to_fun_args(ex.left, undefined, undefined, ex.right);
+            } else if (ex instanceof AST_Assign && ex.left instanceof AST_Symbol) {
+                return new AST_SymbolFunarg({
+                    name: ex.left.name,
+                    default: ex.right,
+                    start: ex.start,
+                    end: ex.end
+                });
             } else {
                 croak("Invalid function parameter", ex.start.line, ex.start.col);
             }
@@ -1037,8 +1042,16 @@ var AST_SymbolConst = DEFNODE("SymbolConst", null, {
     $documentation: "A constant declaration"
 }, AST_SymbolDeclaration);
 
-var AST_SymbolFunarg = DEFNODE("SymbolFunarg", null, {
+var AST_SymbolFunarg = DEFNODE("SymbolFunarg", "default", {
     $documentation: "Symbol naming a function argument",
+    $propdoc: {
+        default: "[AST_Expression] The default for this parameter. For example, `= 6`"
+    },
+    _walk: function (visitor) {
+        return visitor._visit(this, function() {
+            if (this.default) this.default._walk(visitor);
+        });
+    }
 }, AST_SymbolVar);
 
 var AST_SymbolDefun = DEFNODE("SymbolDefun", null, {
diff --git a/lib/output.js b/lib/output.js
index 0cb387ce..066c1432 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -1292,7 +1292,7 @@ function OutputStream(options) {
     DEFPRINT(AST_Symbol, function (self, output) {
         self._do_print(output);
     });
-    DEFPRINT(AST_SymbolDeclaration, function(self, output){
+    DEFPRINT(AST_SymbolFunarg, function(self, output){
         self._do_print(output);
         if (self.default) {
             output.space();