diff --git a/doc/src/sgml/syntax.sgml b/doc/src/sgml/syntax.sgml
index 0b1e5c4e2f0..7a94870b649 100644
--- a/doc/src/sgml/syntax.sgml
+++ b/doc/src/sgml/syntax.sgml
@@ -2373,6 +2373,13 @@ SELECT concat_lower_or_upper('Hello', 'World', uppercase := true);
     having numerous parameters that have default values, named or mixed
     notation can save a great deal of writing and reduce chances for error.
    </para>
+
+   <note>
+    <para>
+     Named and mixed call notations can currently be used only with regular
+     functions, not with aggregate functions or window functions.
+    </para>
+   </note>
   </sect2>
  </sect1>
 
diff --git a/src/backend/catalog/pg_proc.c b/src/backend/catalog/pg_proc.c
index ccec1102d0b..b985d63d77b 100644
--- a/src/backend/catalog/pg_proc.c
+++ b/src/backend/catalog/pg_proc.c
@@ -284,6 +284,12 @@ ProcedureCreate(const char *procedureName,
 		}
 	}
 
+	/* Guard against a case the planner doesn't handle yet */
+	if (isWindowFunc && parameterDefaults != NIL)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("window functions cannot have default arguments")));
+
 	/*
 	 * All seems OK; prepare the data to be inserted into pg_proc.
 	 */