diff --git a/internal/domain/error.go b/internal/domain/error.go
index 3a8e92a..28ab7b5 100644
--- a/internal/domain/error.go
+++ b/internal/domain/error.go
@@ -3,6 +3,7 @@ package domain
 import (
 	"fmt"
 	"strconv"
+	"strings"
 
 	http "github.com/valyala/fasthttp"
 	"golang.org/x/xerrors"
@@ -185,11 +186,41 @@ var (
 	}
 )
 
+var ErrErrorCodeUnknown error = NewError(ErrorCodeInvalidRequest, "unknown error code", "")
+
+//nolint: gochecknoglobals // maps cannot be constants
+var uidsErrorCodes = map[string]ErrorCode{
+	ErrorCodeAccessDenied.uid:            ErrorCodeAccessDenied,
+	ErrorCodeInsufficientScope.uid:       ErrorCodeInsufficientScope,
+	ErrorCodeInvalidClient.uid:           ErrorCodeInvalidClient,
+	ErrorCodeInvalidGrant.uid:            ErrorCodeInvalidGrant,
+	ErrorCodeInvalidRequest.uid:          ErrorCodeInvalidRequest,
+	ErrorCodeInvalidScope.uid:            ErrorCodeInvalidScope,
+	ErrorCodeInvalidToken.uid:            ErrorCodeInvalidToken,
+	ErrorCodeServerError.uid:             ErrorCodeServerError,
+	ErrorCodeTemporarilyUnavailable.uid:  ErrorCodeTemporarilyUnavailable,
+	ErrorCodeUnauthorizedClient.uid:      ErrorCodeUnauthorizedClient,
+	ErrorCodeUnsupportedGrantType.uid:    ErrorCodeUnsupportedGrantType,
+	ErrorCodeUnsupportedResponseType.uid: ErrorCodeUnsupportedResponseType,
+}
+
 // String returns a string representation of the error code.
 func (ec ErrorCode) String() string {
 	return ec.uid
 }
 
+// UnmarshalForm implements custom unmarshler for form values.
+func (ec *ErrorCode) UnmarshalForm(v []byte) error {
+	code, ok := uidsErrorCodes[strings.ToLower(string(v))]
+	if !ok {
+		return fmt.Errorf("UnmarshalForm: %w", ErrErrorCodeUnknown)
+	}
+
+	*ec = code
+
+	return nil
+}
+
 // MarshalJSON encodes the error code into its string representation in JSON.
 func (ec ErrorCode) MarshalJSON() ([]byte, error) {
 	return []byte(strconv.QuoteToASCII(ec.uid)), nil
diff --git a/internal/domain/error_test.go b/internal/domain/error_test.go
index 9d4282b..3214940 100644
--- a/internal/domain/error_test.go
+++ b/internal/domain/error_test.go
@@ -2,6 +2,7 @@ package domain_test
 
 import (
 	"fmt"
+	"testing"
 
 	"source.toby3d.me/website/indieauth/internal/domain"
 )
@@ -10,3 +11,18 @@ func ExampleNewError() {
 	fmt.Printf("%v", domain.NewError(domain.ErrorCodeInvalidRequest, "client_id MUST be provided", ""))
 	// Output: invalid_request: client_id MUST be provided
 }
+
+func TestErrorCode_UnmarshalForm(t *testing.T) {
+	t.Parallel()
+
+	input := []byte("access_denied")
+	result := domain.ErrorCodeUndefined
+
+	if err := result.UnmarshalForm(input); err != nil {
+		t.Fatalf("%+v", err)
+	}
+
+	if result != domain.ErrorCodeAccessDenied {
+		t.Errorf("UnmarshalForm(%s) = %v, want %v", input, result, domain.ErrorCodeAccessDenied)
+	}
+}