diff --git a/sniffio/_impl.py b/sniffio/_impl.py
index c1a7bbf..428695c 100644
--- a/sniffio/_impl.py
+++ b/sniffio/_impl.py
@@ -64,11 +64,13 @@ async def generic_sleep(seconds):
 
     """
     value = thread_local.name
-    if value is not None:
+    # Empty / non-str overrides are programmer errors; treat as unset so we
+    # keep sniffing instead of returning a false library name.
+    if isinstance(value, str) and value:
         return value
 
     value = current_async_library_cvar.get()
-    if value is not None:
+    if isinstance(value, str) and value:
         return value
 
     # Need to sniff for asyncio
diff --git a/sniffio/_tests/test_sniffio.py b/sniffio/_tests/test_sniffio.py
index 02945a9..5d5cf65 100644
--- a/sniffio/_tests/test_sniffio.py
+++ b/sniffio/_tests/test_sniffio.py
@@ -82,3 +82,26 @@ async def this_is_curio():
 
     with pytest.raises(AsyncLibraryNotFoundError):
         current_async_library()
+
+
+def test_empty_name_ignored():
+    with pytest.raises(AsyncLibraryNotFoundError):
+        current_async_library()
+    old_name, thread_local.name = thread_local.name, ""
+    try:
+        with pytest.raises(AsyncLibraryNotFoundError):
+            current_async_library()
+    finally:
+        thread_local.name = old_name
+    token = current_async_library_cvar.set("")
+    try:
+        with pytest.raises(AsyncLibraryNotFoundError):
+            current_async_library()
+    finally:
+        current_async_library_cvar.reset(token)
+    old_name, thread_local.name = thread_local.name, 123  # type: ignore[assignment]
+    try:
+        with pytest.raises(AsyncLibraryNotFoundError):
+            current_async_library()
+    finally:
+        thread_local.name = old_name