diff --git a/package/mongoose/mongoose.mk b/package/mongoose/mongoose.mk
index 8d06ed4689..0693f7553a 100644
--- a/package/mongoose/mongoose.mk
+++ b/package/mongoose/mongoose.mk
@@ -13,7 +13,7 @@ MONGOOSE_INSTALL_STAGING = YES
 # static library
 MONGOOSE_INSTALL_TARGET = NO
 
-MONGOOSE_CFLAGS = $(TARGET_CFLAGS)
+MONGOOSE_CFLAGS = $(TARGET_CFLAGS) -std=gnu99
 
 ifeq ($(BR2_PACKAGE_OPENSSL),y)
 MONGOOSE_DEPENDENCIES += openssl