diff --git a/src/demangle.cc b/src/demangle.cc
index 8e5189d..8f4db36 100644
--- a/src/demangle.cc
+++ b/src/demangle.cc
@@ -37,6 +37,7 @@
 #include "demangle.h"
 
 #include <algorithm>
+#include <cassert>
 #include <cstdlib>
 #include <limits>
 
@@ -1351,10 +1352,11 @@ bool Demangle(const char* mangled, char* out, size_t out_size) {
   }
 
   if (out_size > 0) {
+    assert(n > 1);
     // n is the size of the allocated buffer, not the length of the string.
     // Therefore, it includes the terminating zero (and possibly additional
     // space).
-    std::size_t copy_size = std::min(n - 1, out_size - 1);
+    std::size_t copy_size = std::min(n, out_size) - 1;
     std::copy_n(unmangled.get(), copy_size, out);
     out[copy_size] = '\0';  // Ensure terminating null if n > out_size
   }