diff --git a/chapter2/nonlinpoisson_code.ipynb b/chapter2/nonlinpoisson_code.ipynb
index b29362aa..56346638 100644
--- a/chapter2/nonlinpoisson_code.ipynb
+++ b/chapter2/nonlinpoisson_code.ipynb
@@ -193,7 +193,7 @@
     "problem.solve()\n",
     "converged = problem.solver.getConvergedReason()\n",
     "num_iter = problem.solver.getIterationNumber()\n",
-    "assert converged > 0, \"Solver did not converge, got {converged}.\"\n",
+    "assert converged > 0, f\"Solver did not converge, got {converged}.\"\n",
     "print(\n",
     "    f\"Solver converged after {num_iter} iterations with converged reason {converged}.\"\n",
     ")"
diff --git a/chapter2/nonlinpoisson_code.py b/chapter2/nonlinpoisson_code.py
index 52a3bde3..c9ec2108 100644
--- a/chapter2/nonlinpoisson_code.py
+++ b/chapter2/nonlinpoisson_code.py
@@ -125,7 +125,7 @@ def u_exact(x):
 problem.solve()
 converged = problem.solver.getConvergedReason()
 num_iter = problem.solver.getIterationNumber()
-assert converged > 0, "Solver did not converge, got {converged}."
+assert converged > 0, f"Solver did not converge, got {converged}."
 print(
     f"Solver converged after {num_iter} iterations with converged reason {converged}."
 )