diff --git a/source/DiskImage.cpp b/source/DiskImage.cpp
index c853c591..96b58206 100644
--- a/source/DiskImage.cpp
+++ b/source/DiskImage.cpp
@@ -81,7 +81,7 @@ ImageError_e ImageOpen(	const std::string & pszImageFilename,
 	if (!bExpectFloppy)
 		return eIMAGE_ERROR_UNSUPPORTED;
 
-	pImageInfo->uNumTracks = sg_DiskImageHelper.GetNumTracksInImage(pImageInfo->pImageType);
+	_ASSERT(pImageInfo->uNumTracks);
 
 	*pWriteProtected = pImageInfo->bWriteProtected;
 
diff --git a/source/DiskImageHelper.cpp b/source/DiskImageHelper.cpp
index 6602c00d..4ca82301 100644
--- a/source/DiskImageHelper.cpp
+++ b/source/DiskImageHelper.cpp
@@ -1891,6 +1891,7 @@ void CImageHelperBase::SetImageInfo(ImageInfo* pImageInfo, FileType_e fileType,
 	pImageInfo->uOffset = dwOffset;
 	pImageInfo->pImageType = pImageType;
 	pImageInfo->uImageSize = dwSize;
+	pImageInfo->uNumTracks = pImageType->m_uNumTracksInImage;// Copy ImageType's m_uNumTracksInImage, which may get trashed by subsequent images in the zip (GH#824)
 }
 
 //-------------------------------------