diff --git a/crates/vm/src/stdlib/_thread.rs b/crates/vm/src/stdlib/_thread.rs
index 7ac20af92..efe4ed302 100644
--- a/crates/vm/src/stdlib/_thread.rs
+++ b/crates/vm/src/stdlib/_thread.rs
@@ -104,28 +104,29 @@ pub(crate) mod _thread {
         }
     }
 
+    fn validate_acquire_args(args: AcquireArgs, vm: &VirtualMachine) -> PyResult<(bool, f64)> {
+        let timeout = args.timeout.to_secs_f64();
+        if !args.blocking && timeout != -1.0 {
+            return Err(vm.new_value_error("can't specify a timeout for a non-blocking call"));
+        }
+        if args.blocking && timeout < 0.0 && timeout != -1.0 {
+            return Err(vm.new_value_error("timeout value must be a non-negative number"));
+        }
+        if timeout > TIMEOUT_MAX {
+            return Err(vm.new_overflow_error("timeout value is too large"));
+        }
+        Ok((args.blocking, timeout))
+    }
+
     macro_rules! acquire_lock_impl {
-        ($mu:expr, $args:expr, $vm:expr) => {{
-            let (mu, args, vm) = ($mu, $args, $vm);
-            let timeout = args.timeout.to_secs_f64();
-            match args.blocking {
+        ($mu:expr, $blocking:expr, $timeout:expr, $vm:expr) => {{
+            let (mu, blocking, timeout, vm) = ($mu, $blocking, $timeout, $vm);
+            match blocking {
                 true if timeout == -1.0 => {
                     vm.allow_threads(|| mu.lock());
                     Ok(true)
                 }
-                true if timeout < 0.0 => {
-                    Err(vm.new_value_error("timeout value must be a non-negative number"))
-                }
-                true => {
-                    if timeout > TIMEOUT_MAX {
-                        return Err(vm.new_overflow_error("timeout value is too large"));
-                    }
-
-                    Ok(vm.allow_threads(|| mu.try_lock_for(Duration::from_secs_f64(timeout))))
-                }
-                false if timeout != -1.0 => {
-                    Err(vm.new_value_error("can't specify a timeout for a non-blocking call"))
-                }
+                true => Ok(vm.allow_threads(|| mu.try_lock_for(Duration::from_secs_f64(timeout)))),
                 false => Ok(mu.try_lock()),
             }
         }};
@@ -166,7 +167,8 @@ pub(crate) mod _thread {
         #[pymethod]
         #[pymethod(name = "acquire_lock")]
         fn acquire(&self, args: AcquireArgs, vm: &VirtualMachine) -> PyResult<bool> {
-            acquire_lock_impl!(&self.mu, args, vm)
+            let (blocking, timeout) = validate_acquire_args(args, vm)?;
+            acquire_lock_impl!(&self.mu, blocking, timeout, vm)
         }
 
         #[pymethod]
@@ -252,6 +254,7 @@ pub(crate) mod _thread {
         #[pymethod]
         #[pymethod(name = "acquire_lock")]
         fn acquire(&self, args: AcquireArgs, vm: &VirtualMachine) -> PyResult<bool> {
+            let (blocking, timeout) = validate_acquire_args(args, vm)?;
             if self.mu.is_owned_by_current_thread() {
                 // Re-entrant acquisition: just increment our count.
                 // parking_lot stays at 1 level; we track recursion ourselves.
@@ -259,7 +262,7 @@ pub(crate) mod _thread {
                     .fetch_add(1, core::sync::atomic::Ordering::Relaxed);
                 return Ok(true);
             }
-            let result = acquire_lock_impl!(&self.mu, args, vm)?;
+            let result = acquire_lock_impl!(&self.mu, blocking, timeout, vm)?;
             if result {
                 self.count.store(1, core::sync::atomic::Ordering::Relaxed);
             }
diff --git a/extra_tests/snippets/builtin_thread.py b/extra_tests/snippets/builtin_thread.py
index e9696cce6..3d12e76cc 100644
--- a/extra_tests/snippets/builtin_thread.py
+++ b/extra_tests/snippets/builtin_thread.py
@@ -1,3 +1,21 @@
 import _thread
 
 assert _thread.TIMEOUT_MAX in [9223372036.0, 4294967.0]
+
+from testutils import assert_raises
+
+# Reentrant calls must validate arguments before incrementing the count.
+for error, kwargs in (
+    (ValueError, {"blocking": False, "timeout": 1}),
+    (ValueError, {"timeout": -2}),
+    (OverflowError, {"timeout": _thread.TIMEOUT_MAX + 1}),
+):
+    lock = _thread.RLock()
+    assert lock.acquire()
+    try:
+        with assert_raises(error):
+            lock.acquire(**kwargs)
+        assert lock._recursion_count() == 1
+    finally:
+        while lock._is_owned():
+            lock.release()