diff --git a/src/Components/Blocks/FencedCode.php b/src/Components/Blocks/FencedCode.php
index 85b1117..dab873c 100644
--- a/src/Components/Blocks/FencedCode.php
+++ b/src/Components/Blocks/FencedCode.php
@@ -58,6 +58,10 @@ final class FencedCode implements ContinuableBlock
     ) {
         $marker = \substr($Context->line()->text(), 0, 1);
 
+        if ($marker !== '`' && $marker !== '~') {
+            return null;
+        }
+
         $openerLength = \strspn($Context->line()->text(), $marker);
 
         if ($openerLength < 3) {
diff --git a/src/Components/Blocks/Rule.php b/src/Components/Blocks/Rule.php
index 1622ff0..893406d 100644
--- a/src/Components/Blocks/Rule.php
+++ b/src/Components/Blocks/Rule.php
@@ -29,6 +29,10 @@ final class Rule implements Block
 
         $marker = \substr($Context->line()->text(), 0, 1);
 
+        if ($marker !== '*' && $marker !== '-' && $marker !== '_') {
+            return null;
+        }
+
         if (
             \substr_count($Context->line()->text(), $marker) >= 3
             && \chop($Context->line()->text(), " \t$marker") === ''