diff --git a/src/xkbtext.c b/src/xkbtext.c
index 6741d1a555aec907bd2e61becbab94bda832c114..91c3c33446164f57e0fbd08773b0473c69ef2a85 100644
--- a/src/xkbtext.c
+++ b/src/xkbtext.c
@@ -733,7 +733,7 @@ XkbStringText(char *str, unsigned format)
             }
             else {
                 *out++ = '0';
-                sprintf(out, "%o", *in);
+                sprintf(out, "%o", (unsigned char)*in);
                 while (*out != '\0')
                     out++;
             }