diff --git a/docs/field-fft.ipynb b/docs/field-fft.ipynb
index 50036412b..507e61233 100644
--- a/docs/field-fft.ipynb
+++ b/docs/field-fft.ipynb
@@ -31,7 +31,7 @@
    "id": "5c652ca9-be64-4cef-8a73-0fa617a668ec",
    "metadata": {},
    "source": [
-    "As an example we study a cuboidal geometry with a helix propagating along the `z` direction with a $7, nm$ wavelength."
+    "As an example we study a cuboidal geometry with a helix propagating along the `z` direction with a $7\,\mathrm{nm}$ wavelength."
    ]
   },
   {