diff --git a/frontends/PreprocessOccam.hs b/frontends/PreprocessOccam.hs
index 2715934..d5844ed 100644
--- a/frontends/PreprocessOccam.hs
+++ b/frontends/PreprocessOccam.hs
@@ -187,7 +187,8 @@ handleDirective m s x
     -- currently we support so few preprocessor directives that this is more
     -- useful.
     lookup s []
-        =  do warnP m WarnUnknownPreprocessorDirective "Unknown preprocessor directive ignored"
+        =  do warnP m WarnUnknownPreprocessorDirective
+                $ "Unknown preprocessor directive ignored: " ++ show s
               return return
     lookup s ((re, func):ds)
         = case matchRegex re s of