diff --git a/ext/liquid_c/block.c b/ext/liquid_c/block.c
index a0703cf1..11c2ecdf 100644
--- a/ext/liquid_c/block.c
+++ b/ext/liquid_c/block.c
@@ -19,10 +19,12 @@ static ID
     intern_parse,
     intern_square_brackets,
     intern_unknown_tag_in_liquid_tag,
-    intern_ivar_nodelist;
+    intern_ivar_nodelist,
+    intern_raise_unknown_tag;
 
 static VALUE tag_registry;
 static VALUE variable_placeholder = Qnil;
+static VALUE cLiquidBlock;
 
 typedef struct parse_context {
     tokenizer_t *tokenizer;
@@ -375,7 +377,9 @@ static VALUE block_body_parse_from_serialize(block_body_t *body, VALUE tokenizer
             VALUE markup = rb_utf8_str_new(tag_markup_header_markup(current_tag), current_tag->markup_len);
 
             VALUE tag_class = rb_funcall(tag_registry, intern_square_brackets, 1, tag_name);
-            assert(RTEST(tag_class));
+            if (!RTEST(tag_class)) {
+                return rb_funcall(cLiquidBlock, intern_raise_unknown_tag, 4, tag_name, Qnil, Qnil, parse_context_obj);
+            }
 
             serialize_parse_context_enter_tag(serialize_context, current_tag);
             VALUE new_tag = rb_funcall(tag_class, intern_parse, 4,
@@ -700,6 +704,7 @@ void liquid_define_block_body()
     intern_square_brackets = rb_intern("[]");
     intern_unknown_tag_in_liquid_tag = rb_intern("unknown_tag_in_liquid_tag");
     intern_ivar_nodelist = rb_intern("@nodelist");
+    intern_raise_unknown_tag = rb_intern("raise_unknown_tag"); 
 
     tag_registry = rb_funcall(cLiquidTemplate, rb_intern("tags"), 0);
     rb_global_variable(&tag_registry);
@@ -707,6 +712,8 @@ void liquid_define_block_body()
     VALUE cLiquidCBlockBody = rb_define_class_under(mLiquidC, "BlockBody", rb_cObject);
     rb_define_alloc_func(cLiquidCBlockBody, block_body_allocate);
 
+    cLiquidBlock = rb_const_get(mLiquid, rb_intern("Block"));
+
     rb_define_method(cLiquidCBlockBody, "initialize", block_body_initialize, 1);
     rb_define_method(cLiquidCBlockBody, "parse", block_body_parse, 2);
     rb_define_method(cLiquidCBlockBody, "freeze", block_body_freeze, 0);