diff --git a/src/zm_utils.cpp b/src/zm_utils.cpp
index bc17e5758..6ea4e0dae 100644
--- a/src/zm_utils.cpp
+++ b/src/zm_utils.cpp
@@ -120,16 +120,18 @@ std::string Join(const StringVector &values, const std::string &delim) {
 std::string stringtf(const char* format, ...) {
   va_list args;
   va_start(args, format);
-  int size = vsnprintf(nullptr, 0, format, args) + 1; // Extra space for '\0'
-  va_end(args);
-
-  if (size <= 0) {
-    throw std::runtime_error("Error during formatting.");
-  }
-
-  std::unique_ptr<char[]> buf(new char[size]);
   va_list args2;
   va_copy(args2, args);
+  int size = vsnprintf(nullptr, 0, format, args);
+  va_end(args);
+
+  if (size < 0) {
+    va_end(args2);
+    throw std::runtime_error("Error during formatting.");
+  }
+  size += 1; // Extra space for '\0'
+
+  std::unique_ptr<char[]> buf(new char[size]);
   vsnprintf(buf.get(), size, format, args2);
   va_end(args2);