diff --git a/docs/markdown/Unit-tests.md b/docs/markdown/Unit-tests.md
index b5d3a1b81..898366095 100644
--- a/docs/markdown/Unit-tests.md
+++ b/docs/markdown/Unit-tests.md
@@ -89,6 +89,10 @@ variable `MESON_TESTTHREADS` like this.
 $ MESON_TESTTHREADS=5 meson test
 ```
 
+Setting `MESON_TESTTHREADS` to 0 enables the default behavior (core
+count), whereas setting an invalid value results in setting the job
+count to 1.
+
 ## Priorities
 
 *(added in version 0.52.0)*
diff --git a/mesonbuild/mtest.py b/mesonbuild/mtest.py
index 9975afa22..c417bc0b3 100644
--- a/mesonbuild/mtest.py
+++ b/mesonbuild/mtest.py
@@ -99,13 +99,17 @@ def uniwidth(s: str) -> int:
 
 def determine_worker_count() -> int:
     varname = 'MESON_TESTTHREADS'
+    num_workers = 0
     if varname in os.environ:
         try:
             num_workers = int(os.environ[varname])
+            if num_workers < 0:
+                raise ValueError
         except ValueError:
             print(f'Invalid value in {varname}, using 1 thread.')
             num_workers = 1
-    else:
+
+    if num_workers == 0:
         try:
             # Fails in some weird environments such as Debian
             # reproducible build.