diff --git a/flag.go b/flag.go
index 7af532e..5f14635 100644
--- a/flag.go
+++ b/flag.go
@@ -621,9 +621,14 @@
 // SetWordSeparators sets a list of strings to be considerered as word
 // separators and normalized for the pruposes of lookups.  For example, if this
 // is set to {"-", "_", "."} then --foo_bar, --foo-bar, and --foo.bar are
-// considered equivalent flags.
+// considered equivalent flags.  This must be called before flags are parsed,
+// and may only be called once.
 func (f *FlagSet) SetWordSeparators(separators []string) {
 	f.wordSeparators = separators
+	for k, v := range f.formal {
+		delete(f.formal, k)
+		f.formal[f.normalizeFlagName(string(k))] = v
+	}
 }
 
 // Parsed returns true if the command-line flags have been parsed.
diff --git a/flag_test.go b/flag_test.go
index 10e6ebb..ca500dd 100644
--- a/flag_test.go
+++ b/flag_test.go
@@ -238,8 +238,9 @@
 	if f.Parsed() {
 		t.Error("f.Parse() = true before Parse")
 	}
-	f.SetWordSeparators([]string{"-", "_"})
 	withDashFlag := f.Bool("with-dash-flag", false, "bool value")
+	// Set this after some flags have been added and before others.
+	f.SetWordSeparators([]string{"-", "_"})
 	withUnderFlag := f.Bool("with_under_flag", false, "bool value")
 	withBothFlag := f.Bool("with-both_flag", false, "bool value")
 	if err := f.Parse(args); err != nil {