diff --git a/path.go b/path.go
index 3e5219b..991e909 100644
--- a/path.go
+++ b/path.go
@@ -2,7 +2,7 @@ package html
 
 import "strings"
 
-// path.go: ParseBlockID extracts the slot sequence from a data-block ID.
+// path.go: ParseBlockID extracts the HLCRF slot sequence from a data-block ID.
 // Example: ParseBlockID("L-0-C-0") returns []byte{'L', 'C'}.
 func ParseBlockID(id string) []byte {
 	if id == "" {
@@ -18,7 +18,12 @@ func ParseBlockID(id string) []byte {
 			if len(part) != 1 {
 				return nil
 			}
-			slots = append(slots, part[0])
+			switch part[0] {
+			case 'H', 'L', 'C', 'R', 'F':
+				slots = append(slots, part[0])
+			default:
+				return nil
+			}
 		} else {
 			if part == "" {
 				return nil
diff --git a/path_test.go b/path_test.go
index e57a1ff..81b9e63 100644
--- a/path_test.go
+++ b/path_test.go
@@ -178,8 +178,8 @@ func TestParseBlockID(t *testing.T) {
 		{"C-0-C-1", []byte{'C', 'C'}},
 		{"H-0", []byte{'H'}},
 		{"C-0-C-0-C-0", []byte{'C', 'C', 'C'}},
-		{"X-0", []byte{'X'}},
-		{"H-0-X-0", []byte{'H', 'X'}},
+		{"X-0", nil},
+		{"H-0-X-0", nil},
 		{"", nil},
 		{"L-1-C-0", []byte{'L', 'C'}},
 		{"L-0-C", nil},