diff --git a/playwright/_impl/_transport.py b/playwright/_impl/_transport.py
index 3cc029e18..5b9557eab 100644
--- a/playwright/_impl/_transport.py
+++ b/playwright/_impl/_transport.py
@@ -22,6 +22,7 @@
 from typing import Callable, Dict, Optional, Union
 
 from playwright._impl._driver import compute_driver_executable, get_driver_env
+from playwright._impl._errors import Error
 from playwright._impl._helper import ParsedMessagePayload
 
 
@@ -75,7 +76,11 @@ def send(self, message: Dict) -> None:
         pass
 
     def serialize_message(self, message: Dict) -> bytes:
-        msg = json.dumps(message)
+        try:
+            # NaN and Infinity are not valid JSON and would crash the driver.
+            msg = json.dumps(message, allow_nan=False)
+        except ValueError as e:
+            raise Error(str(e)) from None
         if "DEBUGP" in os.environ:  # pragma: no cover
             print("\x1b[32mSEND>\x1b[0m", json.dumps(message, indent=2))
         return msg.encode()
diff --git a/tests/async/test_assertions.py b/tests/async/test_assertions.py
index 05b35c630..9c31b12af 100644
--- a/tests/async/test_assertions.py
+++ b/tests/async/test_assertions.py
@@ -1218,3 +1218,13 @@ async def test_assertions_should_include_aria_snapshot_in_separate_section(
     actual_line = message.split("\n")[1]
     assert 'heading "Page Heading"' not in actual_line
     assert message.index('heading "Page Heading"') > message.index("Aria snapshot:")
+
+
+@pytest.mark.parametrize("timeout", [float("inf"), float("-inf"), float("nan")])
+async def test_assertions_should_reject_non_finite_timeout(
+    page: Page, timeout: float
+) -> None:
+    with pytest.raises(Error, match="Out of range float values are not JSON"):
+        await expect(page.locator("div")).to_be_visible(timeout=timeout)
+    # The driver connection must survive the rejected call.
+    assert await page.evaluate("1 + 1") == 2
diff --git a/tests/sync/test_assertions.py b/tests/sync/test_assertions.py
index 5180c2461..90cf5a3be 100644
--- a/tests/sync/test_assertions.py
+++ b/tests/sync/test_assertions.py
@@ -1123,3 +1123,13 @@ def test_assertions_should_include_aria_snapshot_in_separate_section(
     actual_line = message.split("\n")[1]
     assert 'heading "Page Heading"' not in actual_line
     assert message.index('heading "Page Heading"') > message.index("Aria snapshot:")
+
+
+@pytest.mark.parametrize("timeout", [float("inf"), float("-inf"), float("nan")])
+def test_assertions_should_reject_non_finite_timeout(
+    page: Page, timeout: float
+) -> None:
+    with pytest.raises(Error, match="Out of range float values are not JSON"):
+        expect(page.locator("div")).to_be_visible(timeout=timeout)
+    # The driver connection must survive the rejected call.
+    assert page.evaluate("1 + 1") == 2