diff --git a/src/util/Alloc.cxx b/src/util/Alloc.cxx
index 115dc96bb..e6f0f25ea 100644
--- a/src/util/Alloc.cxx
+++ b/src/util/Alloc.cxx
@@ -88,8 +88,7 @@ t_xstrcatdup(Args&&... args)
 	const size_t total = FillLengths(lengths, args...);
 
 	char *p = (char *)xalloc(total + 1);
-	StringCat(p, lengths, args...);
-	p[total] = 0;
+	*StringCat(p, lengths, args...) = 0;
 	return p;
 }
 
diff --git a/src/util/ConcatString.hxx b/src/util/ConcatString.hxx
index e3a6a0ad0..abc6dd59b 100644
--- a/src/util/ConcatString.hxx
+++ b/src/util/ConcatString.hxx
@@ -49,18 +49,18 @@ FillLengths(size_t *lengths, const char *a)
 }
 
 template<typename... Args>
-void
+char *
 StringCat(char *p, const size_t *lengths, const char *a, Args&&... args)
 {
-	StringCat(p, lengths, a);
-	StringCat(p + *lengths, lengths + 1, args...);
+	return StringCat(StringCat(p, lengths, a),
+			 lengths + 1, args...);
 }
 
 template<>
-void
+char *
 StringCat(char *p, const size_t *lengths, const char *a)
 {
-	std::copy_n(a, *lengths, p);
+	return std::copy_n(a, *lengths, p);
 }
 
 #endif