diff --git a/core/config.mk b/core/config.mk
index 76e3a33..4b5a12c 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -33,7 +33,7 @@
 
 ifneq ($(GREP_OPTIONS),)
 $(warning The build system needs unmodified output of grep.)
-$(error Please unset your $GREP_OPTIONS.)
+$(error Please unset your $$GREP_OPTIONS.)
 endif
 
 # Standard source directories.