diff --git a/common/include/villas/utils.hpp b/common/include/villas/utils.hpp
index ce6defa42..99f59a2cf 100644
--- a/common/include/villas/utils.hpp
+++ b/common/include/villas/utils.hpp
@@ -163,11 +163,11 @@ char *strcatf(char **dest, const char *fmt, ...)
     __attribute__((format(printf, 2, 3)));
 
 // Variadic version of strcatf()
-char *vstrcatf(char **dest, const char *fmt, va_list va)
+char *vstrcatf(char **dest, const char *fmt, va_list &va)
     __attribute__((format(printf, 2, 0)));
 
 char *strf(const char *fmt, ...);
-char *vstrf(const char *fmt, va_list va);
+char *vstrf(const char *fmt, va_list &va);
 
 // Allocate and copy memory.
 void *memdup(const void *src, size_t bytes);
diff --git a/common/lib/utils.cpp b/common/lib/utils.cpp
index 61666029a..e5422a882 100644
--- a/common/lib/utils.cpp
+++ b/common/lib/utils.cpp
@@ -194,10 +194,24 @@ double boxMuller(float m, float s) {
 
 double randf() { return (double)random() / RAND_MAX; }
 
-char *vstrcatf(char **dest, const char *fmt, va_list ap) {
+char *vstrcatf(char **dest, const char *fmt, va_list &ap) {
   char *tmp;
   int n = *dest ? strlen(*dest) : 0;
-  int i = vasprintf(&tmp, fmt, ap);
+
+  va_list va_copy;
+  va_copy(va_copy, ap);
+  int i = vasprintf(&tmp, fmt, va_copy);
+  va_end(va_copy);
+
+  // Advance va_list
+  const char *format = fmt;
+  if (strcmp(format, "%ju") == 0) {
+    va_arg(ap, unsigned int);
+  } else if (strcmp(format, "%lf") == 0) {
+    va_arg(ap, double);
+  } else if (strcmp(format, "%s") == 0) {
+    va_arg(ap, char *);
+  }
 
   *dest = (char *)(realloc(*dest, n + i + 1));
   if (*dest != nullptr)
@@ -228,7 +242,7 @@ char *strf(const char *fmt, ...) {
   return buf;
 }
 
-char *vstrf(const char *fmt, va_list va) {
+char *vstrf(const char *fmt, va_list &va) {
   char *buf = nullptr;
 
   vstrcatf(&buf, fmt, va);