diff --git a/Makefile b/Makefile
index 52f649f77..77dd3c2e7 100644
--- a/Makefile
+++ b/Makefile
@@ -96,6 +96,9 @@ endif
 endif
 
 libs :
+ifeq ($(CORE), UNKOWN)
+	$(error OpenBLAS: Detecting CPU failed. Please set TARGET explicitly, e.g. make TARGET=your_cpu_target. Please read README for the detail.)
+endif
 	-ln -fs $(LIBNAME) libopenblas.$(LIBSUFFIX)
 	for d in $(SUBDIRS) ; \
 	do if test -d $$d; then \