diff --git a/build-aux/build-ports.sh b/build-aux/build-ports.sh
index 98caf26e..3debb026 100755
--- a/build-aux/build-ports.sh
+++ b/build-aux/build-ports.sh
@@ -49,7 +49,7 @@ if [ -z "${PORTS_CFLAGS+x}" ]; then PORTS_CFLAGS="$PORTS_OPTLEVEL"; fi
 if [ -z "${PORTS_CXXFLAGS+x}" ]; then PORTS_CXXFLAGS="$PORTS_OPTLEVEL"; fi
 if [ -z "${CFLAGS+x}" ]; then CFLAGS="$PORTS_CFLAGS"; fi
 if [ -z "${CXXFLAGS+x}" ]; then CXXFLAGS="$PORTS_CXXFLAGS"; fi
-CFLAGS="$CFLAGS -Werror=format -Wno-error=format-contains-nul"
+CFLAGS="$CFLAGS -Werror=format -Wno-error=format-contains-nul -Werror=implicit-function-declaration"
 CXXFLAGS="$CXXFLAGS -Werror=format -Wno-error=format-contains-nul"
 export CFLAGS
 export CXXFLAGS