diff --git a/AK/StringBuilder.cpp b/AK/StringBuilder.cpp
index 8eb7479aea..08776dd3f3 100644
--- a/AK/StringBuilder.cpp
+++ b/AK/StringBuilder.cpp
@@ -52,6 +52,7 @@ void StringBuilder::appendf(const char* fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);
+    will_append(strlen(fmt));
     appendvf(fmt, ap);
     va_end(ap);
 }