diff --git a/libs/libzrtp/configure.in b/libs/libzrtp/configure.in
index c15ae7cd1c..b29c5c3211 100644
--- a/libs/libzrtp/configure.in
+++ b/libs/libzrtp/configure.in
@@ -35,7 +35,7 @@ esac
 AM_INIT_AUTOMAKE
 AX_PREFIX_CONFIG_H(include/zrtp_config_unix.h,ZRTP,config/config.h)
 
-CFLAGS="$CFLAGS -Wall -Wextra -Wno-unused-parameter -fno-strict-aliasing -fPIC -DZRTP_AUTOMAKE=1"
+CFLAGS="$CFLAGS -std=c99 -Wall -Wextra -Wno-unused-parameter -fno-strict-aliasing -fPIC -DZRTP_AUTOMAKE=1"
 
 # Configuring external libraries
 echo "========================= configuring bnlib =============================="