2 files changed, 8 insertions, 0 deletions
diff --git a/os/log.c b/os/log.c
index 9f95743b5..3dc675f5c 100644
--- a/os/log.c
+++ b/os/log.c
@@ -461,6 +461,7 @@ pnprintf(char *string, size_t size, const char *f, va_list args)
             string[s_idx++] = '%';
             break;
         default:
+            BUG_WARN_MSG(f[f_idx], "Unsupported printf directive '%c'\n", f[f_idx]);
             va_arg(args, char*);
             string[s_idx++] = '%';
             if (s_idx < size - 1)
diff --git a/test/signal-logging.c b/test/signal-logging.c
index b45d8d4f3..f6bc85ce8 100644
--- a/test/signal-logging.c
+++ b/test/signal-logging.c
@@ -206,6 +206,13 @@ static void logging_format(void)
     read_log_msg(logmsg);
     assert(strcmp(logmsg, "(EE) test a\n") == 0);
 
+    /* something unsupported % */
+    LogMessageVerbSigSafe(X_ERROR, -1, "test %Q\n");
+    read_log_msg(logmsg);
+    assert(strstr(logmsg, "BUG") != NULL);
+    LogMessageVerbSigSafe(X_ERROR, -1, "\n");
+    fseek(f, 0, SEEK_END);
+
     /* string substitution */
     LogMessageVerbSigSafe(X_ERROR, -1, "%s\n", "substituted string");
     read_log_msg(logmsg);