diff --git a/src/buffer.c b/src/buffer.c
index 4a27e57..2ddb285 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -89,6 +89,10 @@ int uc_gbuf_printf(GBuf *buf, const char *fmt, ...)
         rc = vsnprintf(gbuf + buf->used, remaining, fmt, apc);
         va_end(apc);
 
+        if (rc == -1) {
+            break;
+        }
+
     } while (rc >= remaining);
 
     if(rc > 0) {
diff --git a/src/dstr.c b/src/dstr.c
index a5ae33f..135d12b 100644
--- a/src/dstr.c
+++ b/src/dstr.c
@@ -279,12 +279,12 @@ int uc_dstr_vsprintf(struct DStr *str, const char *restrict fmt, va_list ap_)
     needed = vsnprintf(str->str + str->len, available, fmt, ap);
     va_end(ap);
 
-    if (needed <= 0) {
+    if (needed < 0) {
         return needed;
     }
 
     if (needed >= available) {
-        rc = uc_dstr_ensure(str, str->len + needed);
+        rc = uc_dstr_ensure(str, str->len + needed + 1);
         if (rc != 0) {
             return -1;
         }