diff --git a/Makefile.detect b/Makefile.detect
index 83994c72..37dba2de 100644
--- a/Makefile.detect
+++ b/Makefile.detect
@@ -110,7 +110,7 @@ endif
 ########################################################################
 # Check for headers (who needs autoconf?)
 
-ifndef $(SEXP_USE_NTP_GETTIME)
+ifndef SEXP_USE_NTP_GETTIME
 SEXP_USE_NTP_GETTIME := $(shell echo "main(){struct ntptimeval n; ntp_gettime(&n);}" | gcc -fsyntax-only -include sys/timex.h -xc - >/dev/null 2>/dev/null && echo 1 || echo 0)
 endif
 
@@ -118,7 +118,7 @@ ifeq ($(SEXP_USE_NTP_GETTIME),1)
 CPPFLAGS += -DSEXP_USE_NTPGETTIME
 endif
 
-ifndef $(SEXP_USE_INTTYPES)
+ifndef SEXP_USE_INTTYPES
 SEXP_USE_INTTYPES := $(shell echo "main(){int_least8_t x;}" | gcc -fsyntax-only -include inttypes.h -xc - >/dev/null 2>/dev/null && echo 1 || echo 0)
 endif