diff --git a/src/black/handle_ipynb_magics.py b/src/black/handle_ipynb_magics.py
index 5807dac..8ae9d2e 100644
--- a/src/black/handle_ipynb_magics.py
+++ b/src/black/handle_ipynb_magics.py
@@ -422,7 +422,7 @@ def visit_Assign(self, node: ast.Assign) -> None:
                     src += f" {args[1]}"
             else:
                 raise AssertionError(
-                    "Unexpected IPython magic {node.value.func.attr!r} found. "
+                    f"Unexpected IPython magic {node.value.func.attr!r} found. "
                     "Please report a bug on https://github.com/psf/black/issues."
                 ) from None
             self.magics[node.value.lineno].append(