diff --git a/panda/Package.pp b/panda/Package.pp
index 52cd2f9344..f4a7a628e4 100644
--- a/panda/Package.pp
+++ b/panda/Package.pp
@@ -40,6 +40,9 @@
   #if $[eq $[PANDA],]
     #error You seem to be attached to some trees, but not PANDA!
   #endif
+  #if $[ne $[canonical $[PANDA]], $[canonical $[TOPDIR]]]
+    #error You are not attached to the right PANDA!
+  #endif
 #else
   #defer PANDA_INSTALL $[unixfilename $[INSTALL_DIR]]
   #defer PANDA_INSTALL_OTHER $[unixfilename $[INSTALL_DIR]]