diff --git a/contrib/pyln-testing/pyln/testing/utils.py b/contrib/pyln-testing/pyln/testing/utils.py
index aee4ec65e..b6729f565 100644
--- a/contrib/pyln-testing/pyln/testing/utils.py
+++ b/contrib/pyln-testing/pyln/testing/utils.py
@@ -792,8 +792,16 @@ class LightningNode(object):
         # Don't run --version on every subdaemon if we're valgrinding and slow.
         if SLOW_MACHINE and VALGRIND:
             self.daemon.opts["dev-no-version-checks"] = None
-        if os.getenv("DEBUG_SUBD"):
-            self.daemon.opts["dev-debugger"] = os.getenv("DEBUG_SUBD")
+        dbgvar = os.getenv("DEBUG_SUBD")
+        if dbgvar and ':' in dbgvar:
+            path = dbgvar.split(':')[0]
+            if lightning_dir.endswith(path + '/'):
+                dbgvar = dbgvar.split(':')[-1]
+            else:
+                dbgvar = None
+
+        if dbgvar:
+            self.daemon.opts["dev-debugger"] = dbgvar
         if os.getenv("DEBUG_LIGHTNINGD"):
             self.daemon.opts["dev-debug-self"] = None
         if valgrind:
diff --git a/doc/contribute-to-core-lightning/testing.md b/doc/contribute-to-core-lightning/testing.md
index 2de963756..f97853732 100644
--- a/doc/contribute-to-core-lightning/testing.md
+++ b/doc/contribute-to-core-lightning/testing.md
@@ -41,7 +41,7 @@ There are four kinds of tests:
 
   `PYTHONPATH=contrib/pyln-client:contrib/pyln-testing:contrib/pyln-proto:contrib/pyln-grpc-proto py.test -v tests/`
 
-  You can also append `-k TESTNAME` to run a single test.  Environment variables `DEBUG_SUBD=<subdaemon>` and `TIMEOUT=<seconds>` can be useful for debugging subdaemons on individual tests, and `DEBUG_LIGHTNINGD` for attaching a debugger to each `lightningd` instance created.
+  You can also append `-k TESTNAME` to run a single test.  Environment variables `DEBUG_SUBD=[<path>:]<subdaemon>` (where path must match the end of the lightning daemon path, for matching only one of several lightningd instances) and `TIMEOUT=<seconds>` can be useful for debugging subdaemons on individual tests, and `DEBUG_LIGHTNINGD` for attaching a debugger to each `lightningd` instance created.
 
   Alternatively, to run a specific test via the `Makefile`, you can specify the test by setting the environment variable `PYTEST_TESTS`: