diff --git a/tools/releasetools/img_from_target_files b/tools/releasetools/img_from_target_files
index 2a0e4cc..871e295 100755
--- a/tools/releasetools/img_from_target_files
+++ b/tools/releasetools/img_from_target_files
@@ -113,7 +113,7 @@
     build_command = ["mkuserimg.sh",
                      os.path.join(OPTIONS.input_tmp, "system"), img.name,
                      OPTIONS.info_dict["fstab"]["/system"].fs_type, "system"]
-    if "system_img" in OPTIONS.info_dict:
+    if "system_size" in OPTIONS.info_dict:
       build_command.append(str(OPTIONS.info_dict["system_size"]))
   else:
     build_command = ["mkyaffs2image", "-f"]