diff --git a/lib/productions/argument.js b/lib/productions/argument.js
index 5292bb2b..f77d4735 100644
--- a/lib/productions/argument.js
+++ b/lib/productions/argument.js
@@ -50,6 +50,10 @@ export class Argument extends Base {
         const message = `Optional dictionary arguments must have a default value of \`{}\`.`;
         yield validationError(this.source, this.tokens.name, this, message);
       }
+      if (this.idlType.nullable) {
+        const message = `Dictionary arguments cannot be nullable.`;
+        yield validationError(this.source, this.tokens.name, this, message);
+      }
     }
   }
 }
diff --git a/test/invalid/baseline/argument-dict-nullable.txt b/test/invalid/baseline/argument-dict-nullable.txt
new file mode 100644
index 00000000..0d0bcef9
--- /dev/null
+++ b/test/invalid/baseline/argument-dict-nullable.txt
@@ -0,0 +1,12 @@
+Validation error at line 13 in argument-dict-nullable.webidl, inside `argument dict`:
+ x2(optional Dict? dict = {})
+                   ^ Dictionary arguments cannot be nullable.
+Validation error at line 14 in argument-dict-nullable.webidl, inside `argument union`:
+boolean or Dict)? union = {})
+                  ^ Dictionary arguments cannot be nullable.
+Validation error at line 15 in argument-dict-nullable.webidl, inside `argument union`:
+ z2(optional Union? union = {})
+                    ^ Dictionary arguments cannot be nullable.
+Validation error at line 16 in argument-dict-nullable.webidl, inside `argument req`:
+  void r(Required? req);
+                   ^ Dictionary arguments cannot be nullable.
diff --git a/test/invalid/idl/argument-dict-nullable.webidl b/test/invalid/idl/argument-dict-nullable.webidl
new file mode 100644
index 00000000..0443fd21
--- /dev/null
+++ b/test/invalid/idl/argument-dict-nullable.webidl
@@ -0,0 +1,17 @@
+dictionary Dict {
+  short x = 0;
+};
+
+dictionary Required {
+  required short x;
+};
+
+typedef (short or Dict) Union;
+
+[Exposed=Window]
+interface X {
+  void x2(optional Dict? dict = {});
+  void y2(optional (boolean or Dict)? union = {});
+  void z2(optional Union? union = {});
+  void r(Required? req);
+};