Sometimes the timestamp is provided as a single double, not a vector

src/R_timestamp.c

@@ -89,8 +89,12 @@ SEXP R_TimestampExt(int64_t ts)
 
 int64_t R_TimestampUnix(SEXP r_ts)
 {
+  if (TYPEOF(r_ts) == REALSXP) {
+    return (int64_t)(Rf_asReal(r_ts) / 1);
+  } else {
     return (int64_t)(Rf_asReal(VECTOR_ELT(r_ts, 0)) / 1);
   }
+}
 
 int64_t R_TimestampExtUnix(SEXP r_ts)
 {