diff --git a/external/Makefile b/external/Makefile
index 0c9f1122f..3a5d84f20 100644
--- a/external/Makefile
+++ b/external/Makefile
@@ -71,7 +71,7 @@ $(TARGET_DIR)/libsecp256k1.% $(TARGET_DIR)/libwallycore.%: $(TARGET_DIR)/libwall
 $(TARGET_DIR)/libwally-core-build/src/libwallycore.% $(TARGET_DIR)/libwally-core-build/src/secp256k1/libsecp256k1.%: $(LIBWALLY_HEADERS) $(LIBSECP_HEADERS)
 	cd external/libwally-core && ./tools/autogen.sh
 	mkdir -p ${TARGET_DIR}/libwally-core-build
-	cd ${TARGET_DIR}/libwally-core-build && PYTHON_VERSION=3 ${TOP}/libwally-core/configure CC="$(CC)" --enable-static=yes $(CROSSCOMPILE_OPTS) --enable-module-recovery --enable-elements --enable-shared=no --prefix=/ --libdir=/ --enable-debug && $(MAKE)
+	cd ${TARGET_DIR}/libwally-core-build && PYTHON_VERSION=3 CFLAGS=-std=c99 ${TOP}/libwally-core/configure CC="$(CC)" --enable-static=yes $(CROSSCOMPILE_OPTS) --enable-module-recovery --enable-elements --enable-shared=no --prefix=/ --libdir=/ --enable-debug && $(MAKE)
 
 external/jsmn/jsmn.h: submodcheck