diff --git a/path.go b/path.go
index 0f3ab9b..f51641e 100644
--- a/path.go
+++ b/path.go
@@ -18,6 +18,9 @@ func ParseBlockID(id string) []byte {
 			if len(part) != 1 {
 				return nil
 			}
+			if _, ok := slotRegistry[part[0]]; !ok {
+				return nil
+			}
 			slots = append(slots, part[0])
 		} else if part != "0" {
 			return nil
diff --git a/path_test.go b/path_test.go
index fda5338..43b24b9 100644
--- a/path_test.go
+++ b/path_test.go
@@ -155,6 +155,8 @@ func TestParseBlockID(t *testing.T) {
 		{"L-1-C-0", nil},
 		{"L-0-C", nil},
 		{"LL-0", nil},
+		{"X-0", nil},
+		{"H-0-X-0", nil},
 	}
 
 	for _, tt := range tests {