diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index a0d1c24e9075..6c2298499777 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -399,8 +399,8 @@ jobs:
       env:
         SYTEST_BRANCH: ${{ github.head_ref }}
         POSTGRES: ${{ matrix.job.postgres && 1}}
-        MULTI_POSTGRES: ${{ (matrix.job.postgres == 'multi-postgres') && 1}}
-        ASYNCIO_REACTOR: ${{ (matrix.job.reactor == 'asyncio') && 1 }}
+        MULTI_POSTGRES: ${{ (matrix.job.postgres == 'multi-postgres') || '' }}
+        ASYNCIO_REACTOR: ${{ (matrix.job.reactor == 'asyncio') || '' }}
         WORKERS: ${{ matrix.job.workers && 1 }}
         BLACKLIST: ${{ matrix.job.workers && 'synapse-blacklist-with-workers' }}
         TOP: ${{ github.workspace }}
diff --git a/changelog.d/15804.bugfix b/changelog.d/15804.bugfix
new file mode 100644
index 000000000000..7c8b954397e4
--- /dev/null
+++ b/changelog.d/15804.bugfix
@@ -0,0 +1 @@
+Fix Sytest environmental variable evaluation in CI.