diff --git a/src/ssb.js.c b/src/ssb.js.c
index 3b9b62fb..8aa42bd0 100644
--- a/src/ssb.js.c
+++ b/src/ssb.js.c
@@ -190,7 +190,7 @@ static JSValue _tf_ssb_addIdentity(JSContext* context, JSValueConst this_val, in
 			}
 			else
 			{
-				result = JS_ThrowInternalError(context, "Unexpected private key size: %zd vs. %d\n", length, crypto_sign_SECRETKEYBYTES);
+				result = JS_ThrowInternalError(context, "Unexpected private key size: %d vs. %d\n", (int)length, crypto_sign_SECRETKEYBYTES);
 			}
 		}
 		else