diff --git a/mongoose.c b/mongoose.c
index 3a2594c9..9a842d90 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -2667,6 +2667,7 @@ size_t mg_vxprintf(void (*out)(char, void *), void *param, const char *fmt,
         is_long++, c = fmt[++i];
         if (c == 'l') is_long++, c = fmt[++i];
       }
+      if (c == 'z') is_long = 1, c = fmt[++i];
       if (c == 'p') x = 1, is_long = 1;
       if (c == 'd' || c == 'u' || c == 'x' || c == 'X' || c == 'p' ||
           c == 'g' || c == 'f') {
diff --git a/src/fmt.c b/src/fmt.c
index 4d197cfe..0f19e691 100644
--- a/src/fmt.c
+++ b/src/fmt.c
@@ -173,6 +173,7 @@ size_t mg_vxprintf(void (*out)(char, void *), void *param, const char *fmt,
         is_long++, c = fmt[++i];
         if (c == 'l') is_long++, c = fmt[++i];
       }
+      if (c == 'z') is_long = 1, c = fmt[++i];
       if (c == 'p') x = 1, is_long = 1;
       if (c == 'd' || c == 'u' || c == 'x' || c == 'X' || c == 'p' ||
           c == 'g' || c == 'f') {
diff --git a/test/unit_test.c b/test/unit_test.c
index ef697646..9f3c9aff 100644
--- a/test/unit_test.c
+++ b/test/unit_test.c
@@ -2321,6 +2321,19 @@ static void test_str(void) {
     ASSERT(chkdbl(mg_str("123e-3"), 0.123));
   }
 
+#if defined(_MSC_VER) && _MSC_VER < 1700
+  // VC98 does not have '%zu'
+#else
+  ASSERT(sn("%zu", 0));
+  ASSERT(sn("%zu", -1));
+  ASSERT(sn("%zu", -2));
+  ASSERT(sn("%zd", 123));
+#if !defined(_MSC_VER)
+  ASSERT(sn("%zd", -3));  // MSVC casts to unsigned
+#endif
+  ASSERT(sn("%zu", 0x1234));
+#endif
+
   ASSERT(sn("%d", 0));
   ASSERT(sn("%d", 1));
   ASSERT(sn("%d", -1));