diff --git a/contrib/src/secp256k1/rules.mak b/contrib/src/secp256k1/rules.mak
index 2b16ced7324798c6c3184b9f06dbd1f4c588d994..7011fca0af7dd7be52cc3469d688dfd165c7dee0 100644
--- a/contrib/src/secp256k1/rules.mak
+++ b/contrib/src/secp256k1/rules.mak
@@ -35,6 +35,6 @@ endif
 .secp256k1: secp256k1 .sum-secp256k1
 	$(RECONF)
 	cd $< && $(HOSTVARS) ./configure $(HOSTCONF) --disable-tests --disable-exhaustive-tests
-	cd $< && $(MAKE) CFLAGS+='$(SECP256K1_CFLAGS)' install
+	cd $< && CFLAGS="$(CFLAGS) $(SECP256K1_CFLAGS)" $(MAKE) install
 
 	touch $@