diff --git a/python_src/lasp/lasp_slm.py b/python_src/lasp/lasp_slm.py
index 9b605dc..63f974b 100644
--- a/python_src/lasp/lasp_slm.py
+++ b/python_src/lasp/lasp_slm.py
@@ -57,6 +57,9 @@ class SLM:
             level_ref_value: Reference value for computing the levels in dB
             offset_t: Offset to be added to output time data [s]
             """
+        
+        if tw == TimeWeighting.ufast:
+            raise RuntimeError('The current implementation of impulse time weigthing is incorrect, as it does not have the proper decay time constant.')
 
         self.fbdesigner = fbdesigner
         if xmin is None and fbdesigner is not None: