diff --git a/src/udev/udevadm-info.c b/src/udev/udevadm-info.c
index 7eec84abd5..ae6d8caf54 100644
--- a/src/udev/udevadm-info.c
+++ b/src/udev/udevadm-info.c
@@ -100,7 +100,7 @@ static int print_all_attributes(sd_device *device, bool is_parent) {
 
                 /* skip nonprintable attributes */
                 len = strlen(value);
-                while (len > 0 && isprint(value[len-1]))
+                while (len > 0 && isprint((unsigned char) value[len-1]))
                         len--;
                 if (len > 0)
                         continue;