log UPDATE avoid returning NULL
diff --git a/src/log.c b/src/log.c
index a63973a..92f7c86 100644
--- a/src/log.c
+++ b/src/log.c
@@ -99,7 +99,7 @@
     }
 
     /* unreachable */
-    return NULL;
+    return "Unknown";
 }
 
 LIBYANG_API_DEF LY_VECODE
@@ -144,7 +144,7 @@
     }
 
     /* unreachable */
-    return NULL;
+    return "Unknown";
 }
 
 LIBYANG_API_DEF const char *