diff --git a/client-sdk/go/config/identifier.go b/client-sdk/go/config/identifier.go
index ac555b91cb..f615d731c6 100644
--- a/client-sdk/go/config/identifier.go
+++ b/client-sdk/go/config/identifier.go
@@ -5,7 +5,7 @@ import (
 	"regexp"
 )
 
-var validID = regexp.MustCompile(`^[a-z0-9_]+$`)
+var validID = regexp.MustCompile(`^[a-z0-9][a-z0-9_]*$`)
 
 // ValidateIdentifier makes sure the given string is a valid identifier.
 func ValidateIdentifier(id string) error {
@@ -14,8 +14,10 @@ func ValidateIdentifier(id string) error {
 		return fmt.Errorf("identifier cannot be empty")
 	case len(id) > 64:
 		return fmt.Errorf("identifier must be less than 64 characters long")
+	case id == "default":
+		return fmt.Errorf("identifier cannot be the string 'default'")
 	case !validID.MatchString(id):
-		return fmt.Errorf("identifier must only contain lower-case letters, numbers and _")
+		return fmt.Errorf("identifier must start with a lower-case letter or number and only contain lower-case letters, numbers and _")
 	default:
 		return nil
 	}
diff --git a/client-sdk/go/config/identifier_test.go b/client-sdk/go/config/identifier_test.go
index f5e3003738..12775a1749 100644
--- a/client-sdk/go/config/identifier_test.go
+++ b/client-sdk/go/config/identifier_test.go
@@ -20,6 +20,11 @@ func TestValidateIdentifier(t *testing.T) {
 		{"222", true},
 		{"abc", true},
 		{"abc_2", true},
+		{"_v", false},
+		{"_foo", false},
+		{"default", false},
+		{"a", true},
+		{"2", true},
 	} {
 		if tc.valid {
 			require.NoError(ValidateIdentifier(tc.id), tc.id)