diff --git a/Modules/_zstd/compressor.c b/Modules/_zstd/compressor.c
index 508b136817872b..45a1dd8da29073 100644
--- a/Modules/_zstd/compressor.c
+++ b/Modules/_zstd/compressor.c
@@ -74,7 +74,7 @@ zstd_contentsize_converter(PyObject *size, unsigned long long *p)
             if (PyErr_ExceptionMatches(PyExc_OverflowError)) {
                 PyErr_Format(PyExc_ValueError,
                              "size argument should be a positive int less "
-                             "than %ull", ZSTD_CONTENTSIZE_ERROR);
+                             "than %llu", ZSTD_CONTENTSIZE_ERROR);
                 return 0;
             }
             return 0;
@@ -83,7 +83,7 @@ zstd_contentsize_converter(PyObject *size, unsigned long long *p)
             *p = ZSTD_CONTENTSIZE_ERROR;
             PyErr_Format(PyExc_ValueError,
                          "size argument should be a positive int less "
-                         "than %ull", ZSTD_CONTENTSIZE_ERROR);
+                         "than %llu", ZSTD_CONTENTSIZE_ERROR);
             return 0;
         }
         *p = pledged_size;