diff --git a/src/bmffimage.cpp b/src/bmffimage.cpp
index b6f6a90f5f..bd7cd3cf7b 100644
--- a/src/bmffimage.cpp
+++ b/src/bmffimage.cpp
@@ -363,8 +363,8 @@ uint64_t BmffImage::boxHandler(std::ostream& out /* = std::cout*/, Exiv2::PrintS
       // Check that the string has a '\0' terminator.
       const char* str = data.c_str(skip);
       const size_t maxlen = data.size() - skip;
-      Internal::enforce(maxlen > 0 && strnlen(str, maxlen) < maxlen, Exiv2::ErrorCode::kerCorruptedMetadata);
       std::string name(str);
+      Internal::enforce(maxlen > 0 && name.size() < maxlen, Exiv2::ErrorCode::kerCorruptedMetadata);
       if (Internal::contains(name, "Exif")) {  // "Exif" or "ExifExif"
         exifID_ = ID;
         id = " *** Exif ***";
diff --git a/src/helper_functions.cpp b/src/helper_functions.cpp
index 736d2a4b14..351de439e0 100644
--- a/src/helper_functions.cpp
+++ b/src/helper_functions.cpp
@@ -13,7 +13,10 @@ std::string string_from_unterminated(const char* data, size_t data_length) {
   if (data_length == 0) {
     return {};
   }
-  const size_t StringLength = strnlen(data, data_length);
+  size_t StringLength = 0;
+  while (StringLength < data_length && data[StringLength] != '\0') {
+    StringLength++;
+  }
   return {data, StringLength};
 }