diff --git a/irdb-libs/libIRDB-core/src/assemblestr.hpp b/irdb-libs/libIRDB-core/src/assemblestr.hpp
index 5e5281d47ad2df9a7b850e6a398c58b961dde83c..dcdc4dfe67df08362b0450ceb34d8f95e16cd963 100644
--- a/irdb-libs/libIRDB-core/src/assemblestr.hpp
+++ b/irdb-libs/libIRDB-core/src/assemblestr.hpp
@@ -11,7 +11,7 @@ static void assemblestr(ks_engine * &ks, IRDB_SDK::Instruction_t *ins, const cha
                 ks_close(ks);
                 char msg[250];
                 snprintf(msg, sizeof msg,
-                         "ERROR: ks_asm() failed during instruction assembly. (instruction=%s, count=%lu, error=%s)",
+                         "ERROR: ks_asm() failed during instruction assembly. (instruction=%s, count=%zu, error=%s)",
                          instruct, count, ks_strerror(error));
                 throw std::runtime_error(msg);
     }