diff --git a/Zend/tests/gh18756.phpt b/Zend/tests/gh18756.phpt
new file mode 100644
index 0000000000000..6e112d9060499
--- /dev/null
+++ b/Zend/tests/gh18756.phpt
@@ -0,0 +1,13 @@
+--TEST--
+Bug GH-18756: Zend MM may delete the main chunk
+--EXTENSIONS--
+zend_test
+--FILE--
+<?php
+
+zend_test_gh18756();
+
+?>
+==DONE==
+--EXPECT--
+==DONE==
diff --git a/Zend/zend_alloc.c b/Zend/zend_alloc.c
index d531270d445ed..6b22fde95ed2d 100644
--- a/Zend/zend_alloc.c
+++ b/Zend/zend_alloc.c
@@ -2047,7 +2047,7 @@ ZEND_API size_t zend_mm_gc(zend_mm_heap *heap)
 				i++;
 			}
 		}
-		if (chunk->free_pages == ZEND_MM_PAGES - ZEND_MM_FIRST_PAGE) {
+		if (chunk->free_pages == ZEND_MM_PAGES - ZEND_MM_FIRST_PAGE && chunk != heap->main_chunk) {
 			zend_mm_chunk *next_chunk = chunk->next;
 
 			zend_mm_delete_chunk(heap, chunk);
diff --git a/ext/zend_test/test.c b/ext/zend_test/test.c
index a7dd604d89ef3..04ece8bd2537e 100644
--- a/ext/zend_test/test.c
+++ b/ext/zend_test/test.c
@@ -1516,3 +1516,13 @@ static PHP_FUNCTION(zend_test_create_throwing_resource)
 	zend_resource *res = zend_register_resource(NULL, le_throwing_resource);
 	ZVAL_RES(return_value, res);
 }
+
+static PHP_FUNCTION(zend_test_gh18756)
+{
+	ZEND_PARSE_PARAMETERS_NONE();
+
+	zend_mm_heap *heap = zend_mm_startup();
+	zend_mm_gc(heap);
+	zend_mm_gc(heap);
+	zend_mm_shutdown(heap, true, false);
+}
diff --git a/ext/zend_test/test.stub.php b/ext/zend_test/test.stub.php
index c9477eef52712..f9cb93b5a1ccb 100644
--- a/ext/zend_test/test.stub.php
+++ b/ext/zend_test/test.stub.php
@@ -262,6 +262,8 @@ function zend_test_cast_fread($stream): void {}
     function zend_test_is_zend_ptr(int $addr): bool {}
 
     function zend_test_log_err_debug(string $str): void {}
+
+    function zend_test_gh18756(): void {}
 }
 
 namespace ZendTestNS {
diff --git a/ext/zend_test/test_arginfo.h b/ext/zend_test/test_arginfo.h
index 5947a6587bbed..c7e3df5c58d24 100644
--- a/ext/zend_test/test_arginfo.h
+++ b/ext/zend_test/test_arginfo.h
@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: 9ddaf4d659226c55d49221c71702fa373d42695e */
+ * Stub hash: 2f161861ab09b6b5b594dc2db7c2c9df49d76aa7 */
 
 ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_zend_test_array_return, 0, 0, IS_ARRAY, 0)
 ZEND_END_ARG_INFO()
@@ -162,6 +162,8 @@ ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_zend_test_log_err_debug, 0, 1, I
 	ZEND_ARG_TYPE_INFO(0, str, IS_STRING, 0)
 ZEND_END_ARG_INFO()
 
+#define arginfo_zend_test_gh18756 arginfo_zend_test_void_return
+
 #define arginfo_ZendTestNS2_namespaced_func arginfo_zend_test_is_pcre_bundled
 
 #define arginfo_ZendTestNS2_namespaced_deprecated_func arginfo_zend_test_void_return
@@ -292,6 +294,7 @@ static ZEND_FUNCTION(zend_test_set_fmode);
 static ZEND_FUNCTION(zend_test_cast_fread);
 static ZEND_FUNCTION(zend_test_is_zend_ptr);
 static ZEND_FUNCTION(zend_test_log_err_debug);
+static ZEND_FUNCTION(zend_test_gh18756);
 static ZEND_FUNCTION(ZendTestNS2_namespaced_func);
 static ZEND_FUNCTION(ZendTestNS2_namespaced_deprecated_func);
 static ZEND_FUNCTION(ZendTestNS2_ZendSubNS_namespaced_func);
@@ -372,6 +375,7 @@ static const zend_function_entry ext_functions[] = {
 	ZEND_FE(zend_test_cast_fread, arginfo_zend_test_cast_fread)
 	ZEND_FE(zend_test_is_zend_ptr, arginfo_zend_test_is_zend_ptr)
 	ZEND_FE(zend_test_log_err_debug, arginfo_zend_test_log_err_debug)
+	ZEND_FE(zend_test_gh18756, arginfo_zend_test_gh18756)
 	ZEND_NS_FALIAS("ZendTestNS2", namespaced_func, ZendTestNS2_namespaced_func, arginfo_ZendTestNS2_namespaced_func)
 	ZEND_NS_DEP_FALIAS("ZendTestNS2", namespaced_deprecated_func, ZendTestNS2_namespaced_deprecated_func, arginfo_ZendTestNS2_namespaced_deprecated_func)
 	ZEND_NS_FALIAS("ZendTestNS2", namespaced_aliased_func, zend_test_void_return, arginfo_ZendTestNS2_namespaced_aliased_func)