diff --git a/modules/core/src/persistence.cpp b/modules/core/src/persistence.cpp
index 6789c78e9d..09435b5850 100644
--- a/modules/core/src/persistence.cpp
+++ b/modules/core/src/persistence.cpp
@@ -817,7 +817,7 @@ char *FileStorage::Impl::gets(size_t maxCount) {
         int delta = (int) strlen(ptr);
         ofs += delta;
         maxCount -= delta;
-        if (ptr[delta - 1] == '\n' || maxCount == 0)
+        if (delta == 0 || ptr[delta - 1] == '\n' || maxCount == 0)
             break;
         if (delta == count)
             buffer.resize((size_t) (buffer.size() * 1.5));
diff --git a/modules/core/test/test_io.cpp b/modules/core/test/test_io.cpp
index 4218cb9297..04426e58f7 100644
--- a/modules/core/test/test_io.cpp
+++ b/modules/core/test/test_io.cpp
@@ -3,6 +3,8 @@
 // of this distribution and at http://opencv.org/license.html.
 #include "test_precomp.hpp"
 
+#include <fstream>
+
 namespace opencv_test { namespace {
 
 static SparseMat cvTsGetRandomSparseMat(int dims, const int* sz, int type,
@@ -799,6 +801,25 @@ TEST(Core_InputOutput, filestorage_base64_basic_memory_JSON)
     test_filestorage_basic(cv::FileStorage::WRITE_BASE64, ".json", true, true);
 }
 
+// issue #21851
+TEST(Core_InputOutput, filestorage_heap_overflow)
+{
+    const ::testing::TestInfo* const test_info = ::testing::UnitTest::GetInstance()->current_test_info();
+    CV_Assert(test_info);
+
+    std::string name = std::string(test_info->test_case_name()) + "--" + test_info->name();
+    const char data[] = {0x00, 0x2f, 0x4a, 0x4a, 0x50, 0x4a, 0x4a };
+
+    std::ofstream file;
+    file.open(name, std::ios_base::binary);
+    assert(file.is_open());
+
+    file.write(data, sizeof(data));
+    file.close();
+
+    // This just shouldn't segfault, otherwise it's fine
+    EXPECT_ANY_THROW(FileStorage(name, FileStorage::READ));
+}
 
 TEST(Core_InputOutput, filestorage_base64_valid_call)
 {