diff --git a/ChangeLog b/ChangeLog
index dfea491beb..7e679724bb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -46,6 +46,10 @@ Changes in version 0.2.0.9-alpha - 2007-10-??
     - Don't try to launch descriptor downloads quite so often when we
       have enough directory information.
 
+  o Minor features (compilation):
+    - Detect non-ASCII platforms (if any still exist) and refuse to
+      build there.  Some of our code assumes that 'A' is 65 and so on.
+
   o Minor bugfixes (directory authorities):
     - Correct the implementation of "download votes by digest."  Bugfix on
       0.2.0.8-alpha.
diff --git a/src/common/compat.h b/src/common/compat.h
index 113399719b..3107727df3 100644
--- a/src/common/compat.h
+++ b/src/common/compat.h
@@ -50,6 +50,10 @@
 #error "It seems your platform does not represent NULL as zero. We can't cope."
 #endif
 
+#if 'a'!=97 || 'z'!=122 || 'A'!=65 || ' '!=32
+#error "It seems that you encode characters in something other than ASCII."
+#endif
+
 /* ===== Compiler compatibility */
 
 /* GCC can check printf types on arbitrary functions. */