diff --git a/tests/test_util_core.py b/tests/test_util_core.py
index 95b97a90..1ff3fb6d 100644
--- a/tests/test_util_core.py
+++ b/tests/test_util_core.py
@@ -40,7 +40,7 @@ def test_str_flag_to_int():
         assert flag == flags[0]
 
     for v in versions:
-        assert v in _flag_cache
+        assert f"BufferUsage.{v}" in _flag_cache
 
 
 if __name__ == "__main__":
diff --git a/wgpu/_coreutils.py b/wgpu/_coreutils.py
index 49485097..1365412e 100644
--- a/wgpu/_coreutils.py
+++ b/wgpu/_coreutils.py
@@ -44,8 +44,13 @@ def error_message_hash(message):
 
 
 def str_flag_to_int(flag, s):
-    """Allow using strings for flags, i.e. 'READ' instead of wgpu.MapMode.READ."""
-    value = _flag_cache.get(s, None)
+    """Allow using strings for flags, i.e. 'READ' instead of wgpu.MapMode.READ.
+    No worries about repeated overhead, because the resuls are cached.
+    """
+    cache_key = (
+        f"{flag._name}.{s}"  # using private attribute, lets call this a friend func
+    )
+    value = _flag_cache.get(cache_key, None)
 
     if value is None:
         parts = [p.strip() for p in s.split("|")]
@@ -61,7 +66,7 @@ def str_flag_to_int(flag, s):
                 value += v
             except KeyError:
                 raise ValueError(f"Invalid flag value for {flag}: '{p}'")
-        _flag_cache[s] = value
+        _flag_cache[cache_key] = value
 
     return value