diff --git a/test/zstring.cpp b/test/zstring.cpp
index 70f4c88..600b493 100644
--- a/test/zstring.cpp
+++ b/test/zstring.cpp
@@ -1,3 +1,4 @@
+#include <fmt/format.h>
 #include <gtest/gtest.h>
 #include <iostream>
 #include <stdplus/zstring.hpp>
@@ -37,6 +38,8 @@
     EXPECT_EQ(str2, zs);
 
     std::cerr << const_zstring(as);
+    EXPECT_EQ("dd", fmt::format("d{}", zs));
+    EXPECT_EQ("dc", fmt::format("d{}", const_zstring(as)));
 
     auto from_str = [&](const_zstring cs) { EXPECT_EQ(cs, "ac"); };
     from_str("ac"s);
diff --git a/test/zstring_view.cpp b/test/zstring_view.cpp
index 84b7cb8..95e9aa5 100644
--- a/test/zstring_view.cpp
+++ b/test/zstring_view.cpp
@@ -1,3 +1,4 @@
+#include <fmt/format.h>
 #include <gtest/gtest.h>
 #include <iostream>
 #include <set>
@@ -49,6 +50,7 @@
     EXPECT_LE(sv, s1);
 
     std::cerr << s1;
+    EXPECT_EQ("bac", fmt::format("b{}", s1));
 
     std::unordered_set<zstring_view> uset{s1, s2};
     EXPECT_EQ(1, uset.count("ac"));