diff --git a/playwright/_impl/_fetch.py b/playwright/_impl/_fetch.py
index 2694ee5e0..3005096eb 100644
--- a/playwright/_impl/_fetch.py
+++ b/playwright/_impl/_fetch.py
@@ -391,11 +391,14 @@ async def _inner_fetch(
     ) -> "APIResponse":
         if self._close_reason:
             raise TargetClosedError(self._close_reason)
-        assert (
+        if (
             (1 if data is not None else 0)
             + (1 if form is not None else 0)
             + (1 if multipart is not None else 0)
-        ) <= 1, "Only one of 'data', 'form' or 'multipart' can be specified"
+        ) > 1:
+            raise AssertionError(
+                "Only one of 'data', 'form' or 'multipart' can be specified"
+            )
         assert (
             maxRedirects is None or maxRedirects >= 0
         ), "'max_redirects' must be greater than or equal to '0'"