diff --git a/bootloader-docker-build.sh b/bootloader-docker-build.sh
index d25e7e0..5a136f0 100755
--- a/bootloader-docker-build.sh
+++ b/bootloader-docker-build.sh
@@ -10,7 +10,7 @@ docker run -t -v $(pwd)/output:/output $IMAGETAG /bin/sh -c "\
 	cd trezor-mcu && \
 	git checkout $FIRMWARETAG && \
 	git submodule update --init && \
-	make -C vendor/libopencm3 && \
+	CFLAGS='-std=c99' make -C vendor/libopencm3 && \
 	export OPTFLAGS=-Os
 	make && \
 	make -C bootloader && \
diff --git a/firmware-docker-build.sh b/firmware-docker-build.sh
index 564a45e..7738a98 100755
--- a/firmware-docker-build.sh
+++ b/firmware-docker-build.sh
@@ -10,7 +10,7 @@ docker run -t -v $(pwd)/output:/output $IMAGETAG /bin/sh -c "\
 	cd trezor-mcu && \
 	git checkout $FIRMWARETAG && \
 	git submodule update --init && \
-	make -C vendor/libopencm3 && \
+	CFLAGS='-std=c99' make -C vendor/libopencm3 && \
 	make && \
 	make -C firmware && \
 	make -C firmware sign && \