diff --git a/test/cpp/src/TestClient.cpp b/test/cpp/src/TestClient.cpp
index 47539dc..a6773d4 100644
--- a/test/cpp/src/TestClient.cpp
+++ b/test/cpp/src/TestClient.cpp
@@ -488,7 +488,7 @@
       testClient.testBinary(bin_result, string(bin_data, 256));
       if (bin_result.size() != 256) {
         printf("}\n*** FAILED ***\n");
-        printf("invalid length: %lu\n", bin_result.size());
+        printf("invalid length: %lu\n", static_cast<long unsigned int>(bin_result.size()));
         return_code |= ERR_BASETYPES;
       } else {
         bool first = true;