diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index bbd6b24..0684014 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -6625,7 +6625,7 @@
     memcpy(cur, data, data_length);
     cur += data_length;
 
-    status = psa_tls12_prf_set_key(prf, pms, cur - pms);
+    status = psa_tls12_prf_set_key(prf, pms, (size_t) (cur - pms));
 
     mbedtls_zeroize_and_free(pms, pms_len);
     return status;