diff --git a/config.c b/config.c
index 7da56345..18f3a5df 100644
--- a/config.c
+++ b/config.c
@@ -724,7 +724,7 @@ value_to_pt_or_px(struct context *ctx, struct pt_or_px *res)
 
         long value = strtol(s, &end, 10);
         if (!(errno == 0 && end == s + len - 2)) {
-            LOG_CONTEXTUAL_ERR("invalid px value (must be of the form 12px)");
+            LOG_CONTEXTUAL_ERR("invalid px value (must be on the form 12px)");
             return false;
         }
         res->pt = 0;