diff --git a/tools/releasetools/build_image.py b/tools/releasetools/build_image.py
index 7567346..f30f787 100755
--- a/tools/releasetools/build_image.py
+++ b/tools/releasetools/build_image.py
@@ -509,9 +509,9 @@
   d = {}
 
   if "build.prop" in glob_dict:
-    timestamp = glob_dict["build.prop"].GetProp("ro.build.date.utc")
-    if timestamp:
-      d["timestamp"] = timestamp
+    bp = glob_dict["build.prop"]
+    if "ro.build.date.utc" in bp:
+      d["timestamp"] = bp["ro.build.date.utc"]
 
   def copy_prop(src_p, dest_p):
     """Copy a property from the global dictionary.