diff --git a/docs/usage.md b/docs/usage.md
index baaf0d4c..2c3d3544 100644
--- a/docs/usage.md
+++ b/docs/usage.md
@@ -144,7 +144,7 @@ tskit refers to a haploid genome, not a diploid individual).
 
 ```{code-cell} ipython3
 inferred_ts = tsinfer.infer(vdata)
-print("Inferred a genetic genealogy for {inferred_ts.num_samples} (haploid) genomes")
+print(f"Inferred a genetic genealogy for {inferred_ts.num_samples} (haploid) genomes")
 ```
 
 And that's it: we now have a fully functional {class}`tskit.TreeSequence`