diff --git a/psutil/_pslinux.py b/psutil/_pslinux.py
index 0bd7f65d1..e19e37a97 100644
--- a/psutil/_pslinux.py
+++ b/psutil/_pslinux.py
@@ -390,7 +390,7 @@ def calculate_avail_vmem(mems):
         slab_reclaimable = mems[b'SReclaimable:']
     except KeyError as err:
         debug(
-            "{} is missing from /proc/meminfo; using an approximation for "
+            "%s is missing from /proc/meminfo; using an approximation for "
             "calculating available memory"
             % err.args[0]
         )