diff --git a/test/msgpack_c.cpp b/test/msgpack_c.cpp
index e17033fb..4029273d 100644
--- a/test/msgpack_c.cpp
+++ b/test/msgpack_c.cpp
@@ -523,7 +523,7 @@ TEST(MSGPACKC, simple_buffer_fixext_1byte_255)
 {
     const size_t size = 255;
     char buf[size];
-    for (size_t i = 0; i != size; ++i) buf[i] = i;
+    for (size_t i = 0; i != size; ++i) buf[i] = (char)i;
 
     msgpack_sbuffer sbuf;
     msgpack_sbuffer_init(&sbuf);
@@ -550,7 +550,7 @@ TEST(MSGPACKC, simple_buffer_fixext_2byte_256)
 {
     const size_t size = 256;
     char buf[size];
-    for (size_t i = 0; i != size; ++i) buf[i] = i;
+    for (size_t i = 0; i != size; ++i) buf[i] = (char)i;
 
     msgpack_sbuffer sbuf;
     msgpack_sbuffer_init(&sbuf);
@@ -577,7 +577,7 @@ TEST(MSGPACKC, simple_buffer_fixext_2byte_65535)
 {
     const size_t size = 65535;
     char buf[size];
-    for (size_t i = 0; i != size; ++i) buf[i] = i;
+    for (size_t i = 0; i != size; ++i) buf[i] = (char)i;
 
     msgpack_sbuffer sbuf;
     msgpack_sbuffer_init(&sbuf);
@@ -604,7 +604,7 @@ TEST(MSGPACKC, simple_buffer_fixext_4byte_65536)
 {
     const size_t size = 65536;
     char buf[size];
-    for (size_t i = 0; i != size; ++i) buf[i] = i;
+    for (size_t i = 0; i != size; ++i) buf[i] = (char)i;
 
     msgpack_sbuffer sbuf;
     msgpack_sbuffer_init(&sbuf);