diff --git a/setup.py b/setup.py
index 37bb2ceb881..2701aa1ce12 100755
--- a/setup.py
+++ b/setup.py
@@ -255,14 +255,16 @@ def _pkg_config(name):
         try:
             command_libs = [command, "--libs-only-L", name]
             command_cflags = [command, "--cflags-only-I", name]
+            stderr = None
             if keep_system:
                 command_libs.append("--keep-system-libs")
                 command_cflags.append("--keep-system-cflags")
+                stderr = subprocess.DEVNULL
             if not DEBUG:
                 command_libs.append("--silence-errors")
                 command_cflags.append("--silence-errors")
             libs = (
-                subprocess.check_output(command_libs)
+                subprocess.check_output(command_libs, stderr=stderr)
                 .decode("utf8")
                 .strip()
                 .replace("-L", "")