diff --git a/src/builtin_function.cpp b/src/builtin_function.cpp
index 94df2c21d..7ba5c9c06 100644
--- a/src/builtin_function.cpp
+++ b/src/builtin_function.cpp
@@ -225,6 +225,10 @@ int builtin_function(parser_t &parser, io_streams_t &streams, const wcstring_lis
     if (argc != optind) {
         if (opts.named_arguments.size()) {
             for (int i = optind; i < argc; i++) {
+                if (!valid_var_name(argv[i])) {
+                    streams.err.append_format(BUILTIN_ERR_VARNAME, cmd, argv[i]);
+                    return STATUS_INVALID_ARGS;
+                }
                 opts.named_arguments.push_back(argv[i]);
             }
         } else {
diff --git a/tests/checks/function.fish b/tests/checks/function.fish
new file mode 100644
index 000000000..51a3b3a32
--- /dev/null
+++ b/tests/checks/function.fish
@@ -0,0 +1,14 @@
+#RUN: %fish %s
+function t --argument-names a b c
+    echo t
+end
+
+function t2 --argument-names a b c --no-scope-shadowing
+    echo t2
+end
+#CHECKERR: function.fish (line {{\d+}}): function: Variable name '--no-scope-shadowing' is not valid. See `help identifiers`.
+#CHECKERR: function t2 --argument-names a b c --no-scope-shadowing
+#CHECKERR: ^
+
+functions -q t2 && echo exists || echo does not exist
+#CHECK: does not exist