diff --git a/pandatool/Package.pp b/pandatool/Package.pp
index 06a4875a1b..9644421ae1 100644
--- a/pandatool/Package.pp
+++ b/pandatool/Package.pp
@@ -42,6 +42,9 @@
   #if $[eq $[PANDATOOL],]
     #error You seem to be attached to some trees, but not PANDATOOL!
   #endif
+  #if $[ne $[canonical $[PANDATOOL]], $[canonical $[TOPDIR]]]
+    #error You are not attached to the right PANDATOOL!
+  #endif
 #else
   #defer PANDATOOL_INSTALL $[unixfilename $[INSTALL_DIR]]
   #defer PANDATOOL_INSTALL_OTHER $[unixfilename $[INSTALL_DIR]]