diff --git a/docs/pages/Source-Types.md b/docs/pages/Source-Types.md
index a855993adff4b7a5b1af3d4ebd3b9cab1dbd7868..56ca3c289d02507bdf2d1e4e6101ec0ca0f5762b 100644
--- a/docs/pages/Source-Types.md
+++ b/docs/pages/Source-Types.md
@@ -37,7 +37,7 @@ for rupture geometry. Truncated Gutenberg-Richter MFDs are used to model grid so
 magnitudes and rates with logic trees of varying a-value, b-value and maximum magnitude. For grid
 sources the relative rate at each grid node is defined using a spatial PDF (see notes on
 spatial PDFs, [below](#spatial-pdfs)). When realizing each source the spatial PDF value is scaled
-by each regional rate in a rate-tree.
+by each regional rate in a required `rate-tree`.
 
 Grid sources are represented in a model using a logic tree with a `rupture-sets.json` defining the
 ruptures on each branch.  Because gridded seismicity models may be governed by regionally