diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8ca2dc5a2..d2cd39021 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,14 +1,18 @@
 # Changelog
 
 This file documents all notable changes to the GCHP wrapper repository starting in version 14.0.0. See also CHANGELOG files for individual submodules, such as:
-- src/GCHP_GridComp/GEOSChem_GridComp/geos-chem/CHANGELOG.md
-- src/GCHP_GridComp/GEOSChem_GridComp/HEMCO/CHANGELOG.md
-- src/GCHP_GridComp/GEOSChem_GridComp/Cloud-J/CHANGELOG.md
-- src/GCHP_GridComp/GEOSChem_GridComp/HETP/CHANGELOG.md
-- src/MAPL/CHANGELOG.md
+- `src/GCHP_GridComp/GEOSChem_GridComp/geos-chem/CHANGELOG.md`
+- `src/GCHP_GridComp/GEOSChem_GridComp/HEMCO/CHANGELOG.md`
+- `src/GCHP_GridComp/GEOSChem_GridComp/Cloud-J/CHANGELOG.md`
+- `src/GCHP_GridComp/GEOSChem_GridComp/HETP/CHANGELOG.md`
+- `src/MAPL/CHANGELOG.md`
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased] - TBD
+### Changed
+- CMake now throws an error if FAST-JX is used with any other mechanism than Hg
+
 ## [14.4.1] - 2024-06-28
 ### Fixed
 - Fixed formatting error in `.github/workflows/stale.yml` that caused the Mark Stale Issues action not to run
diff --git a/src/GCHP_GridComp/GEOSChem_GridComp/CMakeLists.txt b/src/GCHP_GridComp/GEOSChem_GridComp/CMakeLists.txt
index 1036e4ec4..48ea0fbb0 100755
--- a/src/GCHP_GridComp/GEOSChem_GridComp/CMakeLists.txt
+++ b/src/GCHP_GridComp/GEOSChem_GridComp/CMakeLists.txt
@@ -147,3 +147,11 @@ gc_pretty_print(VARIABLE RRTMG IS_BOOLEAN)
 gc_pretty_print(VARIABLE GTMM IS_BOOLEAN)
 gc_pretty_print(VARIABLE LUO_WETDEP IS_BOOLEAN)
 gc_pretty_print(VARIABLE FASTJX IS_BOOLEAN)
+
+# Throw an error unless we are using the Hg mechanism,
+# The fullchem & custom mechanisms now use Cloud-J!
+if(${FASTJX})
+  if(NOT ${MECH} MATCHES "Hg")
+    message(FATAL_ERROR "FASTJX can only be used with the Hg mechanism!")
+  endif()
+endif()