diff --git a/setup.py b/setup.py
index cf99ec3aaaf..8859d0a9367 100644
--- a/setup.py
+++ b/setup.py
@@ -216,7 +216,7 @@ class cmake_build_ext(build_ext):
         # `num_jobs` is either the value of the MAX_JOBS environment variable
         # (if defined) or the number of CPUs available.
         num_jobs = envs.MAX_JOBS
-        if num_jobs is not None:
+        if isinstance(num_jobs, str) and num_jobs.isnumeric():
             num_jobs = int(num_jobs)
             logger.info("Using MAX_JOBS=%d as the number of jobs.", num_jobs)
         else: