diff --git a/clientcredentials/clientcredentials.go b/clientcredentials/clientcredentials.go
index 4afb631..c4e840d 100644
--- a/clientcredentials/clientcredentials.go
+++ b/clientcredentials/clientcredentials.go
@@ -82,7 +82,9 @@
 func (c *tokenSource) Token() (*oauth2.Token, error) {
 	v := url.Values{
 		"grant_type": {"client_credentials"},
-		"scope":      internal.CondVal(strings.Join(c.conf.Scopes, " ")),
+	}
+	if len(c.conf.Scopes) > 0 {
+		v.Set("scope", strings.Join(c.conf.Scopes, " "))
 	}
 	for k, p := range c.conf.EndpointParams {
 		if _, ok := v[k]; ok {
diff --git a/internal/oauth2.go b/internal/oauth2.go
index 1aa51be..fc63fca 100644
--- a/internal/oauth2.go
+++ b/internal/oauth2.go
@@ -35,10 +35,3 @@
 	}
 	return parsed, nil
 }
-
-func CondVal(v string) []string {
-	if v == "" {
-		return nil
-	}
-	return []string{v}
-}
diff --git a/oauth2.go b/oauth2.go
index 4bafe87..6266950 100644
--- a/oauth2.go
+++ b/oauth2.go
@@ -129,9 +129,16 @@
 	v := url.Values{
 		"response_type": {"code"},
 		"client_id":     {c.ClientID},
-		"redirect_uri":  internal.CondVal(c.RedirectURL),
-		"scope":         internal.CondVal(strings.Join(c.Scopes, " ")),
-		"state":         internal.CondVal(state),
+	}
+	if c.RedirectURL != "" {
+		v.Set("redirect_uri", c.RedirectURL)
+	}
+	if len(c.Scopes) > 0 {
+		v.Set("scope", strings.Join(c.Scopes, " "))
+	}
+	if state != "" {
+		// TODO(light): Docs say never to omit state; don't allow empty.
+		v.Set("state", state)
 	}
 	for _, opt := range opts {
 		opt.setValue(v)
@@ -157,12 +164,15 @@
 // The HTTP client to use is derived from the context.
 // If nil, http.DefaultClient is used.
 func (c *Config) PasswordCredentialsToken(ctx context.Context, username, password string) (*Token, error) {
-	return retrieveToken(ctx, c, url.Values{
+	v := url.Values{
 		"grant_type": {"password"},
 		"username":   {username},
 		"password":   {password},
-		"scope":      internal.CondVal(strings.Join(c.Scopes, " ")),
-	})
+	}
+	if len(c.Scopes) > 0 {
+		v.Set("scope", strings.Join(c.Scopes, " "))
+	}
+	return retrieveToken(ctx, c, v)
 }
 
 // Exchange converts an authorization code into a token.
@@ -176,11 +186,14 @@
 // The code will be in the *http.Request.FormValue("code"). Before
 // calling Exchange, be sure to validate FormValue("state").
 func (c *Config) Exchange(ctx context.Context, code string) (*Token, error) {
-	return retrieveToken(ctx, c, url.Values{
-		"grant_type":   {"authorization_code"},
-		"code":         {code},
-		"redirect_uri": internal.CondVal(c.RedirectURL),
-	})
+	v := url.Values{
+		"grant_type": {"authorization_code"},
+		"code":       {code},
+	}
+	if c.RedirectURL != "" {
+		v.Set("redirect_uri", c.RedirectURL)
+	}
+	return retrieveToken(ctx, c, v)
 }
 
 // Client returns an HTTP client using the provided token.