diff --git a/include/pam_authenticate.hpp b/include/pam_authenticate.hpp
index f211a29..1469aef 100644
--- a/include/pam_authenticate.hpp
+++ b/include/pam_authenticate.hpp
@@ -25,7 +25,7 @@
     std::strcpy(pass, appPass);
 
     *resp = reinterpret_cast<pam_response*>(
-        calloc(numMsg, sizeof(struct pam_response)));
+        calloc(static_cast<size_t>(numMsg), sizeof(struct pam_response)));
 
     if (resp == nullptr)
     {