diff --git a/src/lib/formats/upd765_dsk.cpp b/src/lib/formats/upd765_dsk.cpp
index 7c82e9b769a..630bd3da172 100644
--- a/src/lib/formats/upd765_dsk.cpp
+++ b/src/lib/formats/upd765_dsk.cpp
@@ -436,7 +436,7 @@ void upd765_format::extract_sectors(floppy_image *image, const format &f, desc_s
 
 		else if(sectors[ds.sector_id].size() < ds.size) {
 			memcpy((void *)ds.data, sectors[ds.sector_id].data(), sectors[ds.sector_id].size());
-			memset((uint8_t *)ds.data + sectors[ds.sector_id].size(), 0, sectors[ds.sector_id].size() - ds.size);
+			memset((uint8_t *)ds.data + sectors[ds.sector_id].size(), 0, ds.size - sectors[ds.sector_id].size());
 
 		} else
 			memcpy((void *)ds.data, sectors[ds.sector_id].data(), ds.size);
diff --git a/src/lib/formats/wd177x_dsk.cpp b/src/lib/formats/wd177x_dsk.cpp
index 5675e3bd5aa..eab69bb5a15 100644
--- a/src/lib/formats/wd177x_dsk.cpp
+++ b/src/lib/formats/wd177x_dsk.cpp
@@ -527,7 +527,7 @@ void wd177x_format::extract_sectors(floppy_image *image, const format &f, desc_s
 
 		else if(sectors[ds.sector_id].size() < ds.size) {
 			memcpy((void *)ds.data, sectors[ds.sector_id].data(), sectors[ds.sector_id].size());
-			memset((uint8_t *)ds.data + sectors[ds.sector_id].size(), 0, sectors[ds.sector_id].size() - ds.size);
+			memset((uint8_t *)ds.data + sectors[ds.sector_id].size(), 0, ds.size - sectors[ds.sector_id].size());
 
 		} else
 			memcpy((void *)ds.data, sectors[ds.sector_id].data(), ds.size);