diff --git a/hw/block/fdc-isa.c b/hw/block/fdc-isa.c
index e43dc532af..796835f57b 100644
--- a/hw/block/fdc-isa.c
+++ b/hw/block/fdc-isa.c
@@ -147,6 +147,8 @@ static void isa_fdc_get_drive_max_chs(FloppyDriveType type, uint8_t *maxc,
             *maxs = fdf->last_sect;
         }
     }
+    /* fd_formats must contain at least one entry per FloppyDriveType */
+    assert(*maxc);
     (*maxc)--;
 }